Rear differential humming noise

Your description sounds a lot like the rear wheel bearing going out in our Sienna prematurely. At least they're covering it for you....
Had a diff go out in an Xterra and it was definitely an unmistakable whine.

 

Got my truck back with a wheel bearing replaced, and so far seems to be fixed. Only thing I noticed on the invoice was that they didn't replace the axel seal sort of stupid since they were taking everything apart and that's recommended, let's hope doesn't start leaking any soon. Anyways thanks for all your input!!

 

Glad you ended up taking it to the dealer. Yeah I hate them too (have my horror stories as well) but sometimes it is best to just do it. Especially when you are under warranty.