After replacing the brake pads on my 97 Dodge Ram 2500,I noticed a hissing noise as the pedal was depressed.I did not use the bleeders during replacement. I used a C-Clamp to depress the brake Pistons. Could this indicate a booster problem.The vehicles stops with no problem, and the pedal is in a normal range.
Although your old rotors may have looked nice and smooth and flat, they aren’t. Did you turn them or replace them?
That sound could be a vacuum leak. Your power brakes likely use vacuum and if there is a leak there, it would make that noise when you use the brakes, even if you are not moving. On the other hand if you only hear the noise when you are moving and it may vary with the rotation of the wheels, then Alan is likely on the right track, and if your rotors are flat and in good shape it could be just needed a little more time to break in.
The vacuum could be a serious issue as under some conditions you could loose the power part of the brakes and they demand a lot more pressure to stop you car if this happens.
If you are unable to determine the answer, then I suggest you have them professionally checked.
If you did not turn or replace the rotors, you really should have. In most cases newer cars are designed with light weight rotors and they are only designed for one set of pads.