If the clunking noise comes from a corner of the vehicle, it could be something like a loose brake caliper. However, if you think it is a transmission noise, check to see if the fluid is due for replacement. Your manual will have the refresh interval for your vehicle.
I see forum members are giving you great feedback so far, and I wanted to quickly chime in to say I'd suggest getting a full diagnosis at a dealership if you're still unable to find a fix for the clunking noise. Feel free to email me anytime at Katie_Lucille@gmexpert.com for further assistance.
Thanks everyone. The noise seems to be related to the shift linkage. The gear selector shaft is moving in and out ,making a noise on the trans case as it moves in and out. only does this in reverse(rolling). If it is put in neutral while still rolling backwards the noise stops. Can even slightly feel it through the shift handle!Any Ideas?