Alright so I have owned my 2007 mustang for over a year now and about 3 months ago I realized that there was a humming noise coming from the rear end at around 50-80 mph only when i have pressure on the gas pedal. When i decelerate the humming noise drastically decreases but is still there just much quieter.

I went to the deal where i purchased my mustang and they told me it was the rear axle bearings. They were going to charge me 800+ dollars to get it fixed there parts and labor..so i went to my mechanic who has done my work for years. The bearings were shot so i had them replaced. Large amount of metal residue on the magnet inside the differential from the old bearings. (This solved the grinding noise)

The humming noise is still there now however and i am stumped as to what it could be. Its extremely noticeable when inside the car only 50+ mph but it is enough to drive me nuts. Any Ideas? All fluids are fresh. Bearings are brand new. Thanks guys!

 

So you had grinding & whining and now you just have whining.
Some of those metal shavings might be from the ring and pinion.
Your gears might be wore down a little bit creating the whine.

 

Yes, I originally had grinding all of the bearings were replaced. When I had the differential taken apart the gears looked pretty good. Ill take it off this weekend and take a better look. The whining is driving me nuts.

 

Yeah I know it sucks. I had 3.73's that developed a coast whine.
From 60 down to 30 it whined. I tried Royal Purple and then Amsoil.
The Amsoil maybe made it a hair quieter but I couldn't stand it either.
The guy that pulled the 3.73's and installed my 4.10's told me that once
it whines and the pattern is wore in that you can't get rid of the whine.
How long has it been since you got the bearings? Maybe you can take it
back to your mechanic and see if he can make any adjustments?

 

It's humming because it doesn't know the words...

Is there any chance that what you're NOW hearing is tire noise? My Eagles have gotten noisier as time (and mileage) goes on.