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I have a 6-speed 2012 135i, and whenever I idle in neutral, the car vibrates and makes a noise that I can only describe as sort of a faint grinding sound (it doesn't actually sound as bad as that description probably makes it seem, though). If I depress the clutch, the noise and vibration go away completely.

The car drives fine, and doesn't make any unusual noises or vibrations when moving.

I'd be interested to hear any opinions before I take it in. Thanks.

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Usually when I hear what you are describing, I put the issue at the front end of the transmission, either with the front bearing or the carrier bearing supporting the input portion of the transmission. Here is my reasoning:

Your description places the noise/vibration when the transmission is engaged in neutral, which means the clutch is engaged with the input shaft spinning, but no real load being applied to the shaft or internal bearings supporting the shaft. A bearing will tend to make the most noise when running that way. When you depress the clutch pedal, the transmission is not being spun by the engine, so the noise would stop at that point.

Your description would preclude the throw-out bearing, because when it is not engaging the clutch fingers, it's not being turned at all. It can only make noise when being used to disengage the clutch.

Until they get really bad, transmission bearings which may be going out don't make a lot of noise when there is a load on them. I believe the reason is because when there is no load but are spinning, they will rattle (best term I can use to describe it) in their races. This would account for the vibration and the noise. When a load is applied to the bearing, the slack is taken up and the rattle goes away.

From what I've read online, the MT6 in the 135i is a pretty stout transmission, being able to withstand a lot of abuse (to the tune of 600+ rear-wheel horsepower). That being said, if you bought this vehicle used, you don't know how it was used (or abused) by the original owner. It could be that it had a bad bearing from the factory. I know with BMW's reputation, that doesn't happen very often, but it is just a mechanical item which wears out over time and is put together with human hands. Anything is possible and can happen. If you are the original owner of the car, you may have some recourse with BMWNA. I guess this also depends on how many miles are on the vehicle in the first place.

When you go to the shop, don't give them your diagnosis as to what may be going on. Let them figure it out. If you do give them your diagnosis, it may cause them to have preconceived notions of what is wrong and bias them in their diagnosis. What I have described is a high dollar fix, so by not giving them any information like what I've stated here, they won't have $$ running through their minds. If you don't get a satisfactory diagnosis, there is nothing saying you cannot take it to another shop for a 2nd opinion. Also, there is nothing saying you have to take it to the BMW dealership, but I would take it to someone who is knowledgeable with BMW's, though.

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  • Great. Thanks so much for the really thorough answer! Apr 6, 2015 at 12:07
  • Amazing answer, thankyou! Very detailed and well written. Appreciate people like you on the internet.
    – Levi
    Jul 15, 2019 at 20:56

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