help with a noise from the back of the car.

I always say do a visual, jack it up and turn the wheels by hand in neutral etc. check the U joints. if you want to run through the gears with the rear up do that while watching underneath. You can idle it in all gears, I wouldn't rev too much. Keep in mind that you are not under load, this is just a visual you may see something loose or bent etc. Look at the spring bushings, shock bushings, check anywhere that there may be play or loose and missing parts. You would see if a tire has a shift belt or bent rim etc. It is a PITA and a lift may be better for this. If anything, you will "get a feel" of any play in the rear, also you will know if it is a Sure Grip or "One legger" Like stated before start simple, make sure tire pressures are set, check wheel lug torque, if it is a spool or tight clutch setup tire pressures may effect it. It is always good to make sure the lug nuts are tight, Check the differential oil level, and if you are ambitious the transmission oil level and make sure you have a pinion snubber installed while you are under there. I know work right? when it is over you will know what you have or at least get the basics covered . I know you just got the car, but don't get yourself too upset until after you look at it and maybe find the noise. It may be a simple fix