help with a noise from the back of the car.

Does this happen in both a straight line and corner? Tight clutches is a sure grip can be grabby around a corner if the friction modifier additive isn't used. Also, broken spider gears in the differential can make all kinds of funky noises. Have you been doing one wheel burnouts or spinning off gravel onto pavement? Also could be a tailpipe banging on so something or a loose rear u joint.