Ok... I've exhausted my self in the past month trying to do research as to what my problem is with the car.
The car is set up like this when the thud is heard.
Just shifted upward durring acceleration. Clutch has been released into the new higher gear... the exact moment when the throttle is touched (provided it was a low RPM shift so no need to match throttle with release of clutch) to accelerate in the new gear... the thud is present.
Drive shaft appears to be lose.. but I do not know the allowable limit on shaft rotation when car is off. When wiggled back and forth... a faint metal on metal sound is heard. Sorta the same as the driving thud... but very faint. Conclusion has been made that it is something dealing with the drive shaft.
Things checked and or replaced....
Center support bearing
Rear subframe bushings
Front diff bolt
All suspension in the rear (as the entire suspesion has been replaced... noise was present before installation of new suspension)
I'm am now lost as to what it could be.
Only thing that I have come to think to be the problem is... the car has 171k miles on it... the input shaft from the tranny into the drive shaft or the part that fits over the shaft from the tranny has been worn. Allowing play between the tranny and drive shaft resulting in the noise and play in the shaft.
Any imput as to what this problem could be are welcome. Please note that I have read anything and almost everything pertaining to thuds, drive shafts, differential movement and tranny problems on bf.c , m3f.c and various other sites.
ahhh the mysterious thud when changing gears.... driveshaft is a good thing to look at, does everything seem tight (bolts etc) im sure its a long shot for the driveshaft to be at fault on this. im thinking it might have something to do with your differential, there is always a bit of play in the diff box, sometimes under lighter loads (usually 1st to 2nd) youll hear a thud sound, apparently is ok for the car, and a bit of play is typical for every car, but factors weigh on the amount of play that you may have , which create the thud sound, older age, a bit more than usual abuse, weather change or warm/cold car, wear and tear, possible problems in the diff itself. I , myself have this thud , but it only happens in the cooler weather, and only 1st to 2nd, i also know a few others have noticed this noise, one guy though did blow up his diff, because im guessing his problem was something on the catostrophic side to begin with , but it didnt take long for it actually explode
I have a similar noise...could it be the trans/ diff fluid? going to flush fluids on my 325 and see if that does it. Only other thing ive read about is the wheel bearings...but sounds like you dont have problems there.
One trick with mysterious noises is to attach a magnet to a small microphone and attach it to some kind of amp like a boom box in the passengers seat. Move the microphone around between test drives until you find the location where the sound is the loudest. It worked great to find a rattle that was driving be crazy. Details of the arrangement can be modified depending on the type of amp available. Small mics are cheap if you don't have one.
A thud in lower gears is quite normal. Or when you put it in 1st from a stop before you re-engage the clutch. If it is pronounced in the higher gears, it could be from ANY loose part in the drivetrain. It seems you checked many of the parts there, but look at the U-joints in the driveshaft next. I think they are non-replacable on our cars... don't have my manual on me.
Aluminum driveshaft in the future???
I have the thud myself... I was concerned until I rode a 02 Z3 and it had a similar thud. Only in 1st & 2nd shifting.. yours seems different where the thud occurs upon throttle. Maybe the driveshaft is not balanced or worse your rear subframe is goin to shit.. goodluck with your search. Keep us updated
UV joints on the shaft are not replaceable... nearest option is to buy a rebuild one as they are around 500 bones...
As for my problem.. I'm gonna take her down next week... and take a look... I'm gonna replace a few other things... once everything checks good again... I'm done with it... a thump is a thump.. I'll just turn the radio up... ignorance is bliss.... once it completely fails... if that be the case... I shall know what the problem is.
I did the same, except with a webcam for visual inspection of my rear suspension. I need new subframe bushings and my right rear LCA was rattling around in the diff carrier and me camber was never solid..... eww. I could see my diff rotating along the axis of the wheels (not the driveshaft!) when I mashed the throttle.... It was actually rather disturbing how much it moved...
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