Throw out bearing, input shaft bearing, or drive shaft support bearing?

Question

I have a '94 BMW 318is manual transmission. I hear a grinding/whirring noise while driving that sounds to me like a bad bearing. I typically hear it when

1. I engage the clutch. The sounds is more pronounced if I don't give it quite enough gas as I start.
2. As I accelerate
3. When downshifting

The noise is typically worse when the engine is in between 2000 and 3000 rpms. I would like to diagnose this problem, but more generally, what are some guidelines for diagnosing bearing problems in the clutch or drivetrain?

Answer 1

Throw out bearings (TOB) usually make noise when only when you press the clutch pedal. The reason for this is, it is the only time it is engaged and actually can make a noise. If you are hearing this particular noise at 2000-3000 rpm when you aren't actuating the clutch, this can be ruled out almost immediately.

Input shaft bearing will make noise most of the time at any engine speed, though can be more pronounced at a given engine speed. It can also make more noise under a given load. A noise from the input shaft bearing will raise and lower with engine speed, and not with vehicle speed. You'll most likely hear it no matter what gear you are in. From your description, this seems most likely your issue.

The drive shaft support bearing will make noises which will raise and fall with vehicle speed. It will make the noise regardless of engine speed. You can be in 3rd gear going 40 mph or 4th gear at 40 mph and should still be able to hear the same noise (it's just an example ... throwing numbers out there). Considering what you've said, I doubt this is your issue.

It may also be one of the carrier bearings within the transmission. The thing is, though, whether a carrier bearing or an input shaft bearing, you'll need to have the transmission pulled to fix it. A good transmission shop should be able to give you the proper diagnosis.

Answer 2

Sounds like the throwout bearing if you're hearing it mainly when you're operating the clutch pedal. Some vehicles do have an inspection port on the gearboxes bell housing, however in a lot of cases, removal of the gearbox and/or bell housing is required.