Quote:
Originally Posted by Redd
Sounds like brake vacuum pump operating. If you hear it very often then your brake vacuum sensor may be on its way out. It's a common issue especially if you have recently done a brake fluid change and accidentally spilled brake fluid on the sensor. Luckily it's a cheap $20 fix. You can buy the sensor separately from the vacuum hose.
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Thanks! Definitely looks like it's the brake vacuum pump. Did a search for that and found others with the same question. So the noise is normal and happens on all (E9x) M3's, and it's just a matter of how frequently it happens? What would you consider 'very often'? I guess I'm just trying to gauge whether my case is normal or not. It usually happens on the first couple startups of the day. I will do more research as well as monitor the car to take note of how frequently it's happening.
FWIW, I did have my brake fluid changed about 7 months ago, shortly after I purchased the car, so you could definitely be right about the sensor. Can't recall if I heard the noise prior to that.
Edit: Upon reading some more threads, I think my situation might be normal, but I will probably end up replacing the sensor if it starts happening more frequently.