I had my ring and pinion and all bearings replaced about a year ago. It was silent until just recently. It now has a very faint noise on accel from 52-58mph. Quiet otherwise. I cannot hear it with the radio and ac on, but with both off, I can hear it. It is not a loud howl or whine. Is some amount of noise normal?

 

I had the same problem. Fixed once just before end of factor warranty, second last summer. The first time they just replaced the ring and pinion, second, ring and pinion, bearings, and whole lot more. About $1800 at the dealer. Luckily I had an extended warranty. Still quite so far.
Have you recently got new tires or rotated them? Possible tires could be making the noise also.

 

I would have it checked out. Rearend problems can often be fixed if caught early enough, but if let go they can destroy a lot of stuff and cost you a ton of money.

 

Took it to local Ford dealer. He said it was pretty much normal amount of noise and that it wouldn't be worth redoing the rearend again as it could end up being even louder. Said if there is noise for a 10mph range, it is considered out of normal operating noise.

 

Probably your bearings howling mine did too, along with the clunking and such. Took the unit out replaced with a posi unit but the bearing was bent for some reason, probably the brakestands i did with the old motor i have no idea. Pretty sure the bearing was howling on the bearing cage. Just a suggestion, not an entirely big project, just messy and the fluid smells so bad...LOL