Whistling in the Brakes on 2002 BMW 325Ci

When the car starts, there is no noise but after a few seconds of driving the whistling begins. Stops when we brake and as soon as the brake is released, it starts again. When we stop and turn off the car, there is a slow release and whistling sound for a few seconds.

What am I fixing now? Just went through all of the air conditioning and over heating problems for a cool $7,000 in repairs.

Asked by for the 2002 BMW 325Ci
the "whistling" noise is coming from a broken Crankcase Vent Valve located under the intake manifold. As you hit the gas and the engine breathes in air it leaks in through the broken vent valve and causes the whistling. It is a common problem and shouldnt cost too much to get fixed.
that is possible too, but far less common.
I had another person tell me it was a leak in my brake booster?
Thank you - I really appreciate the input.