X308, 3.2L, 1998/99, AJ27, no VVT / transmission: ZF 5hp24
New ATF, new ATF filter, new engine oil, new timing kit.

So I successfully swapped the timing chains, guides and tensioners AND pulled all the pins from the tensioners.

I still have to do the write-up about it. Initially I did that, because of the 5-10 second rattle at startup. Everybody will say, it is because of the tensioners...
But while the guides and tensioners definitely needed replacing, it apparently was not that bad yet, that the noise actually came from there.

Since there are only a few seconds per day where I could locate the source of the noise, I am not too sure yet, where it comes from, but I am fairly sure the noise does not come from the timing chains (I used a "mechanics statoscope" - a big screwdriver): That would kill the theory about new, but "bad out of the box"-tensioners...

Presently I am undecided, if the noise comes from the bellhousing or the pistons. Maybe any bearings?

I found this on youtube - that's how it sounds:
My rattle is not that bad, and definitely not that long, but that's the sound, which disappears after 5-10 seconds...

Any ideas?

 

In video it is not an rattle. Way worse. Does it come underneath?

 

Yes, I think from underneath. I have to check again tomorrow. It is only when I start the car after it stand overnight.
In another thread "asdf" came up with the idea that it could be an exhaust-rattle, That is kind of possible. When I was lying under the car with the car being started, I also had the feeling it came from the exhaust, but there was nothing loose on the exhaust, that could rattle, but if that rattle comes from inside of the exhaust, I would not see anything loose...

There is not much time each day trying to locate the rattle, as it stops very soon after startup (not as in the video from youtube).

 

that’s a piston slapathon

 

Was thinking the same thing. My 01 XJR makes this noise as well, though not as loudly as the one in the video. Sounds very much like the piston slap in my old Chevy pickup and only is audible at stone cold startup for maybe 60-90 seconds. Been doing it since overnight ambient temps dropped from long sleeve shirt to jacket weather. I'll be interested to see if it disappears in May.

 

A piston slap marathon? Well, I'll also keep an open mind for piston slap - if so, then as I have been mentioning, only for a few secs, thus no marathon...
But a piston slap would be a noise coming from the upper region of the engine - my noise is more in the lower region.

I read meanwhile about a lot of various potential faults in the transmission 5hp24 (blown apart A-clutch drum, cracked upper side of valve body), but I found no hint, which noise/rattle would be associated with those and other transmission issues. My transmission works fine, though...
So can anybody think of any transmission fault, which sounds kind of like the sound-bit above I found on youtube?

 

Also, I was thinking: If it were piston slap, this would mean that at least one piston ring does not seal properly anymore. And this would mean that oil gets into the combustion chamber (Cylinder). And this would mean that oil get burned. And this would mean that I would smell that at the exhaust and see gray or blue exhaust fumes. But none of that is visible, and I only had my nose basically stuck in the exhausts a few hours ago - all good.

 

Did you try engaging a gear while you have the rattle? Any change in sound?

 

Piston slap is not straight foward indicator of piston rings not sealing up. Some engines slaps pistons years and years without any trouble. Especially in freezing temperatures. Have you driven the car after all of your work ? If not. Drive it with all gears etc and then listen up again.

 

Its summer in australia though, so temps should be very high, not low.

Have you checked your oil pressure at cold start?

 

To aid in diagnosing piston slap, put a block heater in it and keep it plugged all night and see if the noise disappears altogether when you start it in the morning. 🤷‍♂️🤷‍♂️

 

Thanks for all the replies - here are the answers:

> Its summer in australia though, so temps should be very high, not low.
Yes, it is basically warm here (not that hot - in the 20s, i.e. about 25°C (=77°F)) day and night, maybe a bit below 20 at night). Temperature does not make the difference - the difference in the morning is that the engine has not been started up for many hours.

> Did you try engaging a gear while you have the rattle? Any change in sound?
Not yet, I only have those few seconds every morning to check something.

> Have you driven the car after all of your work ? If not. Drive it with all gears etc and then listen up again.
I am not one, who would get "Frequent Driver Miles", if there were something like that... but I drove a couple of times to the nearby town since the repairs.
I am going thru the gears anyway (my wife thinks I am crazy, as I drive an automatic more like a manual, and she asked already my mechanic about that, but he had no concerns about that... (I drove Manuals only for nearly 30 years before I drove my first automatic)). That noise is really only there at initial startup after many hours of sitting there. Otherwise it sounds fine.

> Have you checked your oil pressure at cold start?
I do not have an oil pressure test kit.

>To aid in diagnosing piston slap, put a block heater in it and keep it plugged all night and see if the noise disappears altogether when you start it in the morning. 🤷‍♂️
It's Australia. The "environment" is my "block heater". So consider the block heater, aka "Australia", as having been applied. The noise is as it is: 5-10 sec. at initial startup like the mad cook in the Muppet Show throwing the pots and pans about.

Insights from today's initial startup with my "stethoscope". aka as big screwdriver (handle pressed to the ear, flat-head pressed to a metal part, which I want to check). Btw.: I can't imagine, that an electronic stethoscope would be any better...
So, not to lose time, I positioned myself already under the car and tested various metal surfaces:
Metal transmission fluid pan: Nothing.
Bell housing: Nothing.
Left catalyst: A little bit
Right catalyst: Quite a lot of racket!
From where I was positioned, I could not reach any engine surface in time before the racket stopped.

In this context I should mention.. - I did not mention it so far, as I did not think this is connected: I have 1 failure code: P1646. This should be the bank 1 upstream oxygen sensor, thus: right side...
I already ordered all 4 oxygen sensors and the oxygen sensor removal tool - let's see. if I manage to get the old ones out, once the stuff arrives...

 

...

 

Regularity, relationship to rpm, bpm, and undertones are attributes I use to diagnose sounds.
Is the sound regular (like locomotive) or irregular (like a bottle of rocks)? Bottle of rocks sound is usually exhaust rattle or broken cats rattling.
Is the sound proportional to rpm as you rev it? Does the sound go away when you rev it? Does it get louder when you rev it?
How quickly does the sound occur? The sound in the linked video is about 330 bpm which indicates that it occurs probably once every 2 revolutions of the engine (assuming idle rpm of around 600-700)
Are there any undertones? I.e. does it sound like just the loud clanking noise at 330 bpm, or does it sound like there are quieter clanking noises between the loud ones?

Load vs no load is also important. In piston slap cases it'll sound louder as the engine revs (more load) compared to when the rev die down (no load).
Tone is important too. Is it more of a click-click (usually valve train) or a clock-clock (usually bottom end) or a scrhh-scrhh (sometimes an air leak or rubbing parts) or an errt-errt (usually transmission or power steering). I know this is silly but it's the best I can describe it without audio.

Best to get a recording of the sound. Leave a memo recorder or phone under the car when you start it. Perhaps perform some tests by giving some rev blips (nothing crazy, 2,000 or less should be sufficient). Note the rpm that the car is at after the cold start so it can be used to compare to the bpm of the sound.

The sound in the video does match the bpm of piston slap, but if you turn the volume way up it sounds a bit more rattly and grindy. And it's certainly not coming from where you'd expect piston slap to come from. Also, as Quadcammer mentioned, you're an oz. With your higher cold start temperatures your pistons would have to be pretty roasted to make any piston slap noises. I could be wrong though. I don't know. The sound almost reminds me of when a friend of mine bought an after market clutch and the bolts too long. Might be torque converter noises but I'm not sure.

Curious, what oil weight do you run?

Next thing is: why does the sound go away in 5-10 seconds. What happens when a car first starts:
oil pressure rises to acceptable levels in 1-2 seconds (oil light should turn off)
oil pressure continues to rise after that as the oil warms up and gets moving
transmission oil pressure rises
idling rpm usually drops down from around 1200 to 1000-900
y'all might be able to add more things to this list

anyways, something that occurs around startup is causing the noise to no longer occur. Pay attention to the gauges as you're listening to the noise, see if it goes away as soon as the idling rpm drops. See if the oil pressure light goes off when it's supposed to.

Okay, that was a lot to read, but ONE MORE THING!

You see that big ol' hole in the bottom of the bell housing. I believe that's an inspection hole that a rubber plug is usually in. You could try to see if taking it off makes the sound much more prominent (indicating some sort of torque converter sound).

 

Oh, as I was typing that out you replied! Muppet show and noise from the right catalyst sure sounds like a bad cat with broken bits getting tumbled around from the exhaust gasses. Although I'm not sure why it would go away after 5-10 seconds, so it's probably not that.
If your repair with the oxygen sensors doesn't work, I'd consider removing the catalyst and seeing if it looks alright in there. You could also start it with the pipe off and see if it still rattles.

That being said, sounds like you may have found your issue, I hope your repair is smooth sailing!

The electronic stethoscope is great for finding sounds that go away quickly if you're working alone. You can clip it on a bunch of components and see which one's loudest. You, however, are so quick on your feet that you don't need one. Impressive that you managed to listen to all those parts in time!

 

Thanks asdf,
yes, our post overlapped.
A few more answers:
The noise start right at startup. I think, revving it up makes it go away faster.
I already thought, engine slap is not very likely, and what you write, would confirm it: It definitely does not get worse when revving up. Plus, the noise is coming more from below than from the upper engine area.

I am not sure, what "oil weight" is, but:
ATF in my 5hp24: Mobile LT71141
Engine Oil: Castrol Edge 5W30
...and while we are at it: Power steering fluid: Nulon SYNATF with Dex III properties

 

The noise reminds me of a similar noise made by a 454 SS Chevelle.
I chased it down to a bad spring on a exhaust Heat Riser Valve and
never heard another thing from it. Funny thing, it only made noise when
the engine was cold.

 

@ Randy: Would you know, if the X308 has such a exhaust Heat Riser Valve , and if so, where it is?

 

Late today, when it was definitely warm already, let's say 77°F, I placed my camera UNDER the X308 (not far from catalyst) and recorded a movie of the startup, well to record the noise. Next I converted the MP4 to an MP3 and cut out the relevant audio section. See attachment "X308 rattle 2". So does this sound like rattle from the catalyst?

PS: I received quite a shock when I wanted to start the car: Display said: "gearbox failure" and the car would not start at all... - Looks like I tricked myself: I was in the process of installing DTRL (daytime running light) on the X308, and to get an ignition-on feed I had pulled the ignition-relay (which I forgot)... - without which there is not starting... - a bit silly, that of all the possible messages I had to get a gearbox failure message due to ignition relay removed...

 

Does not seem to change the rattle frequency with rpm. Likely coming from the catalytic converter. Perhaps you should by a mechanic's stethoscope to verify.