What is this noise??

[monteoutside]

I recently bought a 2002 Sport Trac with an existing noise that the original owner says has always been there. It is loudest at around 40 MPH and is heard ONLY when accelerating. When coasting it disappears. It sounds very similar to road noise as if I drove big knobby tires (which I don't) or like added differential noise as if the truck were driving in 4 wheel drive (which it's not).
This is the only Sport Trac I've ever driven. Is this really normal?

 

[MyTrac]

same noise on my end. But I have so many noises now I can't even imagine trying to figure em all out. Just make sure you are hyper vigilant about what else is going on at the same time....report back? What other info can u give us?

 

[Markaprice73]

like this? http://www.youtube.com/watch?v=wHIoVlwra0c about 40mph
This was mine with bad differential bearings. It started as a quiet roar/hum. Then a year or so turned into the noise you hear in the vid. Had the diff rebuilt. Now my ST is as quiet as a Prius http://www.youtube.com/watch?v=5SO0lO-aURY

 

[600remguy]

U joints is where I would start.

 

[monteoutside]

Markaprice73, thanks for the video links, they were helpful. I believe my problem is the same thing though your noise was farther along. I had a mechanic ride along to listen and he said the same thing: rear diff bearings are shot.

 

[Markaprice73]

No problem. Good luck. They claimed they had to rebuild my entire diff. Maybe they can catch yours before screwing up other stuff. But, my guess is they rebuilt the entire diff(bearings/ring/pinion/spider gears/seals/crush sleeve/shims/axle bearings..) so they could warranty the work. Fine by me. I'm just a little surprised as i thought these Ford 8.8's were supposed to last A LOT longer. My diff has never been under water and the vent tune has never been clogged. I will never again believe for one second when they say the diff fluid doesn't ever need to be changed.