I was able to pull the passengers side cam cover today and look inside - everything looked pretty spotless and the timing was dead on, at least the two cams were relative to each other. I did realize after goosing it a bit at idle that I could get the noise to happen - both in the earlier RPM range of 1.8-2k while revs increased, and it would happen again momentarily as revs were dropping, in the 2.5k range. I got a couple videos - it's also a bit easier to hear when I hold the phone underneath the car.

Starting to suspect those cats a bit more... It's a bit difficult to get to them to jiggle them though. I'll bring a stethoscope to work tomorrow and get a coworker to rev it while I listen. It'd be impressively loud if it was the cats after all, but I guess they are just big metal cans after all, if rang just right... I guess it'd be a relief too, although those cats are NLA so I'd need to get aftermarket cats welded in?

quick video, underneath the car:
https://drive.google.com/file/d/13gg...ew?usp=sharing
longer video:
https://drive.google.com/file/d/1lmU...ew?usp=sharing

 

Sounds like a timing or fuel knock to me. A vacuum leak could cause it as the computer tries to compensate for the mixture.

I would spray starting fluid or carb cleaner around the intake system to see if the rpm's change. Maybe the knock sensors are bad? Fuel trim readings from an OBD sensor can also diagnose a vacuum leak.

Mine had a knock/rattle sound coming off of idle which turned out to be caused by a vacuum leak. The vacuum leak was in the part load breather tube coming off the left front cam cover area. Took forever to find it, ended up replacing the part load breather tube. It has never ran this good!

Just some random thoughts. I'll be watching to see what it ends up being. Good luck.

 

I had my friend rev it while I poked around with the stethoscope - oddly I never found a place where I could hear it in the stethoscope, even though I could "feel" the rattle by hand being transmitted through the exhaust. I listened under the car and the rattle seems to be coming from the first muffler (the combined one for both banks) or the sleeve joints immediately after it. When we were testing it I noticed a bit of water dripping out from those sleeve joints every time it made the rattle. I'm guessing some water got in there and froze/thawed enough times to maybe cause a crack or gap to happen.

I checked the clamp nuts and both were tight. I loosened them and tried to wiggle the joints slightly just to slide it around and maybe seat the pipe slightly better, then retightened. Wasn't able to do great on both of those since the car wasn't on jacks or a lift, just doing what I could by reaching. I did have that section of exhaust apart a while ago, so maybe something happened back then.

It seems to be much reduced now, and much less consistent. Hopefully that is the issue and it wasn't just a coincedence. I can still hear it when revving at park and doors open, but it doesn't sound like something is about to eat itself anymore, and it's barely noticeable during driving now.

I did get the chassis ears, but with the weather being in the teens over the past week I haven't been able to work on it. It should warm up this week or next, so I will report back once I try the ears or pull it apart. I'll wire brush any crud off the sleeves and clean them up, maybe some kind of antiseize if that is something recommended for those sleeve joints? I'll also try to get a look inside the first muffler in case something is loose or broken in there.

 

I did have that section of exhaust apart a while ago, so maybe something happened back then."

Double check your work; it sounds like you are getting an exhaust "rattle" that you get with a pin hole leak . . .

 

I verified with the chassis ears that the sound was most concentrated around that sleeve joint - I tested the pass side cat, "bridge" between the two pipes near the flex joints and transmission, and the sleeve joint, and the sleeve joint mic was the only one picking up a rattling. I disconnected the second mufflers and didn't see any obvious cracking or contamination on the sleeve. Also, it was a bit hard to hear over the rumble, but I could still slightly hear the rattling while revving the engine with the secondary mufflers disconnected. I shone a light down the muffler and didn't see anything obvious dangling in there... Ran out of daylight at that point, guess the next thing to do is drop that first muffler and inspect it more closely on a bench.