Anti-lock brake problem


My 2003 Pontiac Aztek makes a hissing sound followed by a loud stuttering noise when I apply the brakes at very low speed. I also feel the thumping in the brake petal. Occasionally the anti lock brake light comes on. This whole sequence of events does not happen consistently-- just about half the times that I am braking at low speed. My mechanic has not been able to figure out the problem.