What Causes "Howling" Rear Gears?

I know some of the things that can cause rearend gears to howl or sing in an annoying way, but I wanted to put this out there for you guys to comment on......

It's a common problem I've run into when restoring old Mopars, and was wondering if we could lay out some of the MOST COMMON causes for rearend noise........(OK comics......stick to the mechanical aspects please).

How about a little input as well on the effectiveness of additives (like the Mopar lube) on actually cutting down on some of the common noise from old or worn rears..........

I'd like to hear what you'se guys have to say in general from your experiences.