I have been experiencing a strange vibration/rumbling in my '02 SMG M3 under hard acceleration. When I'm hard on the throttle (mostly in gears 1-3) there is a rough vibration from the rear of the car, I want to say it's from the rightish side but that could potentially be my imagination (definitely rear though). The rumble typically only lasts 300-500 RPM under hard acceleration (which goes by very quickly), it occurs somewhere in the 3500-4500 RPM range. If I am accelerating slowly through this RPM range it does not seem to occur. It is like a hard stutter, almost like a series of ABS pulses or some sort of slip/grab. This rumble/stutter seems to have gotten more severe recently.
The Diff oil was changed (with the friction modifier) maybe 15,000 miles ago (when I bought the car). After doing some reading on various forums I was pretty convinced that the rumbling issue was caused by the central support bearing or the guibo, so I had both replaced last week. Sadly this did not resolve the issue.
Other random information (not sure if any of it's relevant): The car has had a noticeable whine from the diff whenever your off the throttle (its been doing it since I bought it more than a year ago), but various people have told me that it's normal for these diffs to make some noise. I had the sub-frame reinforced with the Turner kit about 6ish months ago. I had the RTABs replaced (stock bushings with limiters) about 3 months ago. The car throws a flat tire warning maybe once every ~3 weeks, I clear it and it goes away. Transmission is SMG and I'm betting the clutch has never been replaced, the car has about 103k miles on it (but it does pass the 6th gear-40mph and floor it test).
I am pretty stumped as to what might be causing this rumble/vibration, but it is really worrying me. Can anyone point me in the right direction for what might be causing this? I really hate taking anything to the stealership, but this problem seems to beyond the knowledge level of my local indys.