If this noise occurs regularly anytime the vehicle is in motion, it could be a frayed inner speedometer cable (assuming that you don't have an electronic speedometer). However, that wouldn't be my first choice. Also, if the speedo cable was that bad so as to make audible noise, I suspect the needle on the speedo would be jittery also.

1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.