Have someone ride in the BACK seat when you drive around. <img src="/forums/images/graemlins/smile.gif" alt="" />

Do you have a winch? if so, the rollers will rattle on a bumpy road.

Is it a metal on metal sound? Squeek? rattle? Plastic? Is it definitely coming from inside the Runner? maybe crack a window next time you hear it and see if it gets louder to make sure it is inside or out.

99 Limited 4Runner 4WD
ARB front Bumper Kennesaw rear bumper
WARN 9500 Winch
Hella 4000s