I had the same thing on mine, sounded like a very light "pop, pop, pop, pop" behind my dash, drove me insane for a few days while I hunted for it and I ended up tracking it back to the check valve in the line that runs from the brake booster to the vacuum pump. Easy check to see if its bad, if you can blow through that line from both ends, the check valve has failed and needs to be replaced (I actually think you are supposed to be able to blow from the booster to towards the pump but not from the pump end towards the booster, if memory servers, either way, if you can blow through both ends, the check valve is bad). If you don't replace it fast enough, you WILL DESTROY your brake booster and have CONSIDERABLY more work and more expense in fixing it.


On EODGuys suggestion and my personal experience, I would check that line and if it leaks down at all when you blow through it - replace it! In my case my "pop, pop, pop" was the check valve letting small "puffs" of air through at a time and me hearing it "flapping", it was actually that loud as it failed.



BMW Vacuum Pipe With Non-Return Valve (X5)

• - SKU: BMW-34336758560