Over the years on this board I have noticed a tendency to go for the worst, when trying to diagnose a "noise".

My advice has always been, start Simple, Easy, and Cheap, when searching for remedies.

After confirming that none of the driveshaft 'wear' components are the source of your noise--- U-Joints, Center bearing---to the point of renewing them all.---easy once you have the driveshaft out. Total cost--a hundred or so dollars for parts.

Drain the rear end---into a light colored or white pan---cat litter plastic box works just fine. Examine the fluid for metallic shards---fine particles.
I doubt you'll find anything out of the ordinary.

****Refill the Rear with a Synthetic
https://www.redlineoil.com/75w90-gl-5-gear-oil.

Cost- litter box, 20 bucks for a quart of the Synthetic gear oil.

Drive It.

Rear ends rumble. Rear wheel bearings rumble.
Volvo RWD Rear End failures are few, unless you run it Dry, or Race it.

But then, if you're looking for a PITA job, go ahead and replace the rear.

Cheers