Well, if a vacuum leak, you should be able to find it by lifting the hood
with the engine running. A vac leak could come from any number of hoses.

The brakes are easily inspected by taking the tires off, but I don't think
it's "normal" for BMWs to have those squeelers on the brakes. But, since the
car was purchased used, and it's a 1999 model, then anything could be true.
You'll have to pull the tires off to see if the brakes are the problem.

My Maxima does the same thing, hissing, under hard acceleration. For me, it
is the exhaust, a leak around one of the gaskets I have to get around to
fixing. I used to get a bee buzzing sound, that was the flex joint in the
exhaust going, before I put a new pipe on. Must have overlooked tightening a
joint.

sweaking????