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