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      05-05-2012, 01:17 AM   #29
Major General

Drives: 340i M-sport AT
Join Date: Mar 2006
Location: Chicago

iTrader: (1)

Ok, I've some more extensive reading and have some diagnostic to add to this that may help others in the future.

So, engine running, trans in neutral.
There is a noise.
Question; is it the throw out bearing or the input shaft bearing?

Things to do to diagnose:
If you LIGHTLY press the clutch pedal and the noise stops, it's likely the TOB.
Why? In neutral, clutch pedal out, the clutch is engaged. The input shaft to the trans is spinning. The TOB sits around the input shaft. If the TOB is out of spec, or not lubed properly, it could rattle around the input shaft.
By pressing lightly on the clutch pedal, pedal NOT pushed in, there is load on the TOB and stabilizes it enough to stop the rattling.

If a light push on the clutch pedal does not stop the noise, then push the clutch pedal all the way in. This will fully disengage the clutch from the flywheel and the input shaft will soon stop spinning. If the noise then stops, it could likely be the input shaft bearing.

The input shaft bearing is a precision piece and has ball bearings in it's construction. The ball bearings could fail or gain greater tolerance between them. As the input shaft spins, it spins those bearings and you'll hear it making noise. Also, the bearing itself can be worn thus creating a larger tolerance that could allow the bearing to rattle around the shaft. The noise created by the input shaft bearing may make a more whirring type sounds. That noise/sound will change with engine speed/rpm. As the engine speed changes so to will the sound/noise of the bad input shaft bearing.

I hope this can help someone in the future with trying to diagnose this noisy problem. The research helped me to understand this issue better.
It also explains why there seem to be 2 different viewpoints on the issue.
If there is a noise and you lightly push on the clutch pedal and the sound stops, then it's likely the TOB. If you have to fully depress the clutch pedal for the sound to stop, then it's likely the input shaft bearing.
Of course this info is for diagnostic and until you actually get in the trans to find the exact problem, you can't be 100% positive. Hopefully this will get you to around 80%.