What Causes "Howling" Rear Gears?

OK, a few things.

worn/ rough/ pitted bearings

worn/ rough/ pitted/ damaged ring and pinion

With original tapered axle bearings, if not adjusted correctly. Won't be long for this world

Metal touching metal. Remember that original spring hangers were rubber bushed. If you install "harder" modern heavy/ race/ "improved" bushings, this may well transmit more noise into the cabin

Various add--on traction devices. Any installed metal links can transmit more noise into the cabin This might even include a pinion snubber if it uses a minimal rubber.

"low" rears (higher numeric) spin faster and are generally more noisy

Don't forget tires---some tires are produce much more road noise than others