There have been several posts over the last few years discussing this topic, described as a 'growling', or 'humming' coming from the rear end at 40-55 mph, and can usually be eliminated by accelerating, decelerating, or sometimes by putting the transmission in sport mode. I chased this phenom for over a year on my 99 XJ8 (100k miles) and finally cured it so I wanted to share the results.

Could not duplicate the noise on a lift. First thought was a bearing or u-joint going out that vibrated only under certain loading conditions... checked u-joints (no play), wheel bearings had slight play at 6 & 12 clock positions, but none noticeable at 3 & 9 positions. Checked diff output bearing which had some play but measured within tolerance. Jurid joint looked good. No play in driveshaft center bearing. Also checked exhaust components for interference with other components or possible internal cat/muffler plate failures. Just couldn't find anything obvious. Took it to a local Jag specialist and they couldn't find anything either... their answer was "wait until it gets worse"...

Started thinking internal differential issues. Found a used IRS at a local junk yard. Swapped the entire rear suspension to eliminate the diff and all the related bearings and u-joints but the noise remained.

Working up the driveline, I started researching transmission issues. Discovered that some Mercedes models also used the 5HP24E and started searching some MB forums. Eventually learned that the Torque Converter Clutch (TCC) in this unit actually has 3 modes (disengaged, slip-mode, and fully engaged) and when the trans fluid gets old it can lose some lubricity characteristics. So when the computer is telling the TCC to 'slip', the old fluid doesn't allow that, and some TCC 'hunting' ensues, causing a vibration, which translates down the driveline and sounds like it's coming from the rear suspension.

My trans fluid had never been changed, so I did a fluid flush and guess what, the noise is gone... I'm sure some of the other suspected components can cause similar symptoms, but the moral of this story is (as most X308 owners now know) that the ZF claim that their fluid is 'good for life' is a crock.

Grady