nah its the bearing. it used to do that w/ mine but i took the seals off each side and then sprayed it w/ brake parts cleaner and then put some in a tupperware to let it soak overnight. then the next day i dryed them w/ a heat gun and then put some hi-temp disc break and bearing grease in it. it spins silent now. something like this was posted above but i thought i would comment on how i did it. learned it from the cleaning instructions on my bones swiss skateboard bearings a few years ago. those things are still fast and smooth as hell after 4 years.

87 22R PU, 2? body lift, 33? BFG muds, 4.88's, F&R Lock-Rights, Rancho Shocks, EB's Street RV Head w/ 268 Cam, LCE Header, 2" to 2 1/4" exhaust, Flowmaster 50 series, Magnaflow high flow cat, complete Herculined SR5 interior, V6 BB, MC, and calipers.