I have a 1998 Nissan 200SX that I'm trying to figure where a noise is coming from, I believe from the rear. The noise it's making only happens while I'm accelerating from 4th and 5th gears and I only hear from inside the car. It's a tap sound kind of a like the tire makes when it crosses over a small groove in the road. Once I get to the speed I need, it makes no sound. Only when accelerating. I checked my tires and they look fine, I've listened out the windows and can hear no sounds from the tires or wheel bearings.
Any ideas? Thanks.

Bushings? Diff Fluid?