danosoo

I have a noise in the drivetrain that thumps about 5 times when I go from 0 to about 10 mph. Sounds a lot like driving on the highway reflectors No vibration, noise or steering issues after that . Checked everything I can think of. Had it running on jack stands, checked underneath for anything rubbing or loose. All the u-joints seem to be tight. Minimal lash in all the shafts, trans and differential. Even sat in the trunk while a friend drove but couldn't hear the thumping from there. About ready to put it on a dyno to get some resistance on the rear wheels. Any suggestions out there about what I might look into ?

habbyguy

A center support bearing can act like that (higher torque of accelerating from a start causing more deflection).

Check the rear differential bushings for play (I just point a camera at them and shift through P-R-N-D and back a couple times, with a bit of accelerator to add some torque).

I had a really odd intermittent vibration that I finally traced down to the exhaust system oscillating (worn rubber hangers). New rubbers fixed it (only found it with the car on stands, all four wheels turning 60mph).

Torque converters can make a noise like that, but I don't recall it happening at that low of a speed (only when the lock-up circuit kicks in/out).

The dyno might work well - you might try putting a cheap "auto-ear" device on some of the likely suspects - on a dyno, you won't have to deal with wind noise, so it might work really well to ID the source.

danosoo

Thanks Habby, Thanks for your input. I looked at the driveline parts online since they where not visible from underneath. Saw the flex and the carrier. Was able to reach the flex and far enough back to shake the driveshaft. It flopped around in the carrier bearing about an inch in every direction. Everything about the noise made sense. The thumping at low speed and the straightening out with higher speed with the centrifugal force. Guess it's like putting a long screw in wit a drill wobbling till the rpms increase. A lot of steps to get it swapped out. Glad I didn't have other plans for the whole day. Had an ah ha moment when I saw the small opening in the bottom of the transmission mount that was just large enough to slip in a wrench to hold the nut on the flex plate bolts to disconnect it. They could have made the driveshaft easier to get apart. Maybe some U bolts on the universal joints rather than having to turn a bolt an eighth inch at a time.

habbyguy

One thing to watch for when you break the shaft to install the new CSB - make sure and mark the two halves so you can put them back together the same way. The shaft (two pieces) is balanced as a single unit, and putting it back together in a different orientation might cause an imbalance. I remember getting the shaft out was fiddly, but compared to some of the other "interesting work" I did on my E61, not too bad. ;-)

danosoo

I did mark them Hap. What I didn't remember to do is put back the thin washer before I put them back together. Got the bolt tight, looked down and there sat the washer. Had to take the bolt out and put it back 1 degree (one click on a ratchet wrench at a time) a second time. Sheesh !

habbyguy

Oh, I hate it when that happens (and yeah, we've all done the same thing).

The good news is that the mistake wasn't made by the 18 year old trainee at your local BMW stealership. He would have looked around, and tossed the washer into the dustbin. ;-)

danosoo

I couldn't live with myself if I did that. Would .haunt me for the rest of my life ! ha ha ha