Likely suspension, if he hears it during while not accelerating or shifting gears. I don't know much about that generation of mustangs but AFAIK an 87 is a RWD car (no CV joints?). It could be a worn u joint, but you'll typically hear that under acceleration or when the car shifts into gear. Could be worn bushings on a sway bar or ball joints/control arms. If you'd like I'll ask in Mustang forum. Does he hear it in the front? on one side more than the other?