I have a howling noise rear end only when I accelerate

go to the magicmechanic website and send him this--hes the best there is and will rply

differential side bearings.....if the noise changes when on and off the gas ...ring and pinion....if a service was done did they add limited slip addative to oil...i hope so...

the bad bearings and axles probably caused damage or wear to the gears / pinion when under load the bearings keep the axles straight and keeps downward pressure from loads off the gears and gear bearings --look at gear mesh will probably be off very slightly and not from backlash shims but from ring gear bearings or pinion bearing damaged

HI if the noise comes and goes when you accelerate or decelerate you may be dealing whit the ring and pinion needs adjustment or is going out, remove the rear cover and check for excess metal shavings,Hope this help

The final drive (diff) could be low or out of oil. Have a look around inside of the rear wheels for any oil leaks. Also ckeck the pinion seal. But get oil into the filler plug before you do anyting else.

I would certainly replace the axle with play. But this may not be all. Could be wheel bearing as well.

Constant howl....usually wheel bearing.
Howl on acceleration.......usually rear pinion bearing (the bigger of the two on the pinion) in the diff (rear axle)
Howl on de-acceleration....the front (smaller) pinion bearing in the diff.(rear axle)

Need a little humor now and then, anyway it sounds like you have a pinion gear bearing that's noisy, they really howl when they get worn out, in order to change this bearing the rear end gears must be removed, new bearings should be installed on all parts, 4 total, the metal from the failed pinion bearing will ruin the rest of the bearings, might as well only do it once, also change the axle bearings and seals while you are in there.

I Am a Mechanic I have mainly worked on diesel for the last 12 years I have a 2004 F-150 FX4 EXT CAB TOW PACKAGE 5,4 4X4 WITH 9.75 rear end. I had the same problem I drained my rear end and put motorcraft and the limited slip addative I got from the ford dealer here in town. SAME PROBLEM. I drained the rear end again. put in amsoil and 2 "I said 2BOTTLES OF LIMITED SLIP ADDITIVE." that was almost 1 yrs ago I havnt had any problems since and I spend about 3/4 of the time pulling a trailer.

As a matter of fact the diff can make noise on and off the gas, my own car a 1996 Corvette does this, it needs the ring and pinion replaced, is this what you are talking about when you say they told you that you needed a whole new differential?