Brake Noise

Bought my 89 Carrera three months ago, 47,000 miles on the clock. My mechanic checked it out and pronounced it sound. However, after braking (the pedal feels fine and no mushiness whatsoever), I hear a noise like air bleeding into something. It comes from the from front. I just had the rears replaced (rotors + pads), but it was happening before. Oh, it doesn't happen all the time and it doesn't happen until the car is warm and I've used the brakes several times.

Any ideas?