Could it be the ABS? The ABS makes a groaning / pounding / howling noise if you were braking hard enough to invoke the ABS.
Although not common on our cars, a similar noise can come from hot spots on the rotors. If it is from hot spots, it tends to get worse as the brakes heat up, and it will be there most of the time. Hot spots cause rotor thickness variation which generates a pounding while braking. The rotors will have a pattern of dark blue or black spots on the friction surface mixed with the normal silver color.
