Truck: 2006 F150 FX4
Mileage: 110,000
2" autospring level, 18" wheels/33" tires

Symptoms:
I have a whirring/humming noise coming from the rear of the vehicle. I can hear it between the cab and the bed. And I don't think it is as far back as the differential. I don't hear it underneath the cab. It is speed dependent, not RPM dependent. I can hear it only when the truck is moving. I can hear it from 0 mph-top speed. It doesn't come and go with speed.

There is no clunking, grinding, ticking, popping, ratcheting, etc. It is a smooth hum. No vibration either. Truck drives fine, but the noise is fairly new and apparent.

I just changed my tranny fluid. There is no problem with the tranny and it is shifting and going into gear just fine.

I checked my u-joints and everything still looks nice and tight.

Not sure what it could be?
Transfer case? Output shaft? Maybe the bearing inside there?

 

Tore the truck apart this weekend. Pulled the diff cover. Pulled the axles out. Replaced the axle bearings, changed the rear brakes (since the back end was apart anyway), changed the drive shaft u-joints front and back. Put is all back together and the noise is still there.

Pretty sure it is a pinion bearing in the front of the diff. Gonna take it to the rear end shop this week and get it looked at.

Once that is done the entire rear of the truck will have been rebuilt.

 

Problem fixed! Rear pinion bearing was going bad. Changed all 4 bearings in the diff and she is all better!

 

Just to be nosy, what was your cost for the pinion bearing replacement? What did they use for rear-end oil?

 

The rear end job was $700 and they replaced it with the required synthetic oil, I don't remember off the top of my head the specs.

Hoopers in Sun Valley, CA. did the work and they are awesome. All they do is rear ends.

 

Got mine done last week as well. Three out of the 4 bearings in the diff were bad. $900 got them all replaced, axle seals, oil and replaced front diff oil while it was in the air. That shouldn't have been a problem in my opinion at 88K miles.

 

hummm.... had a similar issue but the dealer (2 different ones) said there was nothing wrong with the bearings in the dif. I changed the bearings and seals on the axel, new u-joints, rear brakes, shocks, bushings. Couldn't get rid of that noise. Only happend on turns, mostly right. I ended up adding "friction modifier" to the rear end and the problem went away, the Ford shop actually told me there was a TSB on it, 10$ only found it at the dealer.
Still thinking about replaceing the bearings in the diff anyway..... 900$, I may wait a while yet.