This noise has been on my truck for a while (I noticed it not long after I bought it). From what I can tell, it only occurs during acceleration and sometimes while just maintaining speed.

For instance, I'll accelerate (normally) up to 40 MPH. Once I get to 40, the noise stays for a little, then it will usually go away after a few seconds. Also, if I let off the gas pedal the noise goes away immediately.

It's hard to describe the noise. Its not a whine or a clunk. Its more like a grinding sort of noise (not sure if it is actually grinding though). It's pretty low pitched, I think the best description I can come up with is if you spray water at the concrete with a fast stream, that sounds pretty much like it.

Im not sure if its a differential problem (my guess) or a tranny problem (I really hope not). Sometime this week I will try to change out my diff fluid (it's been leaking so it's probably low anyway).

Where is this tag that tells me what weight fluid to use?
Do I need friction modifier for an open diff or is that just for LS diffs?
How tight do the diff bolts need to be? I dont have a torque wrench.
Im assuming all I do to change the fluid is remove the cover, let all the fluid drain out, replace the cover (with gasket or RTV silicone), refill it?

Thanks.

 

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Carrier bearing noises usually don't go away when the differential is loaded or unloaded. The noise will transfer from side to side if you pitch the truck left and right a little while going down the road, since you're shifting the carrier around in the housing.

If they're really bad, pinion bearings make noise all the time. If not, you'll hear them when the differential is loaded up, as in accelerating. Unbolt the rear driveshaft and spin the pinion flange by hand, see if there's any binding or grabbing.