Here's the bottom line with the transmission "clunk".
Firstly it is very common in the "earlier BMW's"..... it was an issue in the e 30's from the 80's across the 3/5 and 7 series.
It cannot be corrected without serious cost, and even then there is no guarantee it will be 100% clunkless.
If you raise your car rear off the floor and start at the wheels - a small movement fwd & back will reveal a little "play"... this means the wheel drive shaft output from the diff has some freedom inside the diff.
Next, if you do the same with the prop shaft, you will also find some free play in that rotation. This is a combination of the diff and a little in the gearbox.
If you add the movement of the prop shaft to the wheel drive shaft movement, you have a fair amount of play. The reason it is so apparent in the first / second gears is the abrupt change from high rpm to to high rpm... yes, high to high rpm.
The motor will slow from 4000 + rpm in 1st to 1000 - rpm in the time it takes you to move the gearshift from 1st to 2nd, you release the clutch, and require the motor to turn at 3000 + rpm in 2nd immediately. This change from deceleration to acceleration firstly must take up the free play in the transmission / drive line.... "clunk"...
BMW fitted a rubber guibo between the gearbox output flange and the propshaft flange. This was intended to "smooth" the power flow to the drive line. Like most rubber components, it has a useful life after which it deteriorates in it's flexibility.
READ THIS LINK ARTICLE...
http://www.pelicanparts.com/bmw/techart ... -Guibo.htm
I know of one guy here who had the diff "re-engineered" so that the tolerances were reduced to microns - it cost him a bomb, and didn't eradicate the issue 100% because the ZF gearbox also has considerable play. He ultimately stripped out the entire BMW gearbox and diff and replaced them with Toyota components before he was satisfied.
So, you can consider these alternatives:- replace the guibo (available from a Meyle stockist) - Use a "heavier" diff oil (not recommended by BMW) - when changing gears, be as "gentle" as you can in the transition from 1st to 2nd... One suggestion which I haven't tried, but which was suggested by BMW, is to fill the rear axle carrier with an expanding high density foam. The carrier is hollow, and amplifies diff noise considerably. The reason this was suggested was because the clunk seemed to be less apparent on M models, and on those cars, that carrier is filled with a "ballast" to weigh down the rear of the car and provide better traction to the wheels given the power from the motor....or sound proof the car so you never hear it again.
Link to rear axle carrier diagram..
http://www.realoem.com/bmw/images/thumb_8mc.png
If you have the car in second gear, try
abruptly depressing and releasing the accelerator repeatedly in small "tap down / release / tap down etc movements with the car in gear and rolling - clunk, clunk, clunk....
You must realise, the diff is literally between the seats under the rear storage box on a vertical line downwards. This means any noise from that area will seem louder and in close proximity to you - annoying isn't it !
I've forgotten about her.
