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Hello All:

Well, I'm going to give a shot at explaining what I feel when driving my 1996 S500 with 124,000 miles. Terms like "clunking", "rattling", "vibrating", etc. are hard to translate over the internet, but lets see if I can describe what I am feeling in such a way that someone on this forum might have an idea as to what might be going on.

Ok, here goes...

Several weeks back, I started to notice a feeling in my forearm as it rested on the center console while I was driving at highway speed (60 to 80MPH). At first, it felt like maybe a solid object was rattling around in my center console and the reverberation was transferring up to my arm. I opened up the center console, took everything out and kept driving. I continued to notice the knocking, rattling, vibrating feeling. It was then that I realized this must be some kind of drivetrain issue.

I continue to feel this and now I can even "hear" it. The rattle/vibration/sensation seems to manifest itself most when I am in more of a coasting mode while driving. I commute 60 miles one way each day, so I have a lot of time to notice when and under what circumstances I feel the rattle. When I am in harder acceleration or am going uphill, the sensation is way less noticeable, nearly gone actually. When I crest the hill and hit the flats, the rattle/vibration comes back slightly. When I have more of a downhill trajectory and the whole drivetrain system "loosens up" on coasting or deceleration, the rattle/vibration is especially noticeable and the noise is quite apparent.

I have thought of things like transmission mounts, driveshaft related parts like U-joints, that funky flex-disk thing or whatever it is that attaches the shaft to the transmission and rear differential. I've also wondered if there is some kind of looseness forming in the rear suspension or differential itself. I had the entire differential switched out about 5 months ago (old one blew up and I located a used one from the guys I bought the car off of), so that caused me to wonder if maybe they missed something or if some of the parts had gotten loose.

The issue does appear to be getting more pronounced, which is concerning me that I may be approaching the time of a catastrophic failure... not something I want to entertain up here in New England as the hard winter approaches.

Clearly, I need to take this in, but I am wondering if anyone here on the forum has any ideas for me so I can point my mechanic in the right direction (I have an Indy shop I take her to).

Thanks!!

rubondfan2
 

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Outstanding Contributor Always Remembered RIP
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This sounds like a prop shaft fault, one of the couplings or centre bearing going down. At the same time do have the engine mounts checked as these can throw the whole drive train out of line.
 
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