Bean,
First, read this:
http://www.bimmer.info/forum/showthread.php?t=14093
Without knowing what kind of car you have/for what market it was produced, we can't give as accurate of an answer.
Here's what I think. If the car has a vacuum brake booster (ie - brake BOMB), you've got a bad valve in it. These units work on the principle of changing the atmospheric pressure on one side of a rubber diaphram, essentially subjecting it to engine vacuum. The other side of the diaphram (that mounts to the firewall and passes through into the driver's footwell area) has a valve designed to quietly exchange air between its chamber and the car's cabin. They can often get clogged with dust and wheeze.
best, whit