i lost the rear end in my old dodge truck, its howled like a banshee till the gears chunk ed up and locked the rear up, took 2 weeks to kill it. i was off road got hung up on some rocks and the gear oil leaked out the pinion seal. left me with no lube, and it died fast. i did refill it on the way home, but it was to late.

so maybe your low on gear oil. end seals leak and such.