I'm reposting this topic from a month or so ago. Have a 2009 535xit with just over 10k last week. A few weeks ago I noticed a sound like high pressure air escaping when stepping on the brakes. The sound appears to eminate from the right rear seat area(suspiciously close to the right rear air shock location).

Brought it to the dealer to have them test it and of course it didn't occur while mechanic was in car. He said there was a common problem with caliper squeaks but this is a definite escaping air under pressure sound. If any one has suggestions or experience with this please let me know. Thanks for your help.

 

Could be small stone trapped between brake pad and brake disk. Try removing wheel (if you got run flats you might not have a jack etc) and have a look see. Might see some scoring on the disk - one side or other. It only takes a small stone to be trapped at the top to make that noise - happened twice during my driving career. Did manage to clear by driving backwards and braking hard once.