If it's making noise, then I suspect there is some damage or wear going on already. Adding oil may stop it from getting worse, but won't fix the problem.

It could be a wheel bearing, ring and pinion wear, or possibly some bearings in the diff that are the culprits.

By all means check the fluid, but I would recommend jacking the rear of the car up and spin each wheel to see if the bearings are the cause of the noise. When they start going south, it's pretty obvious with the spin check.

I speak with some experience as I have recently replaced my rear wheel bearings, and just had my diff rebuilt.