could be any number of things...
whats it steer like ?
got any popping noise ?
one side more than the other ?
more promenent noise when stopping
or starting or TURNING ?
only happens when your foots on the brake ?
how many miles on the brake pads ? (check the linings?)
how many miles on the shocks ?
ball joints ever been replaced ?
checked the sway bar bushings ?
manual hubs or semi-auto (are you
sure the hubs are unlocked?)