I have 2002 fx4 with 165k. A little back story. Winter here in Colorado gives me a good opportunity to work on the truck. Last weekend I changed rear diff fluid due to limited slip clutches binding while accelerating through turns. Fluid was pretty bad but could tell po serviced it before(felpro gasket was used instead of rtv). I used correct fluid and friction modifier while changing. It’s been 200miles or so. Now I am hearing an intermittent whine or high pitched humming noise through acceleration but only when I really get on the accelerator or am under load going up hill. Then after the truck is warmed up it gets much quieter. It sounds like it’s coming from the rear but I am not 100% sure. Maybe it’s unrelated to the rear diff?

ok second noise, I suspect this is a wheel bearing and/or bad tire noise in the front. Around 20mph I start hearing wom wom wom wom all the way through to 50mph. The noise slightly goes away when I turn right but definitely still there. The tires are BFg ko2 a/t and have 35k miles on them. They have worn a little uneven due to negligence, mountain driving, and older suspension components but not in horrible condition. Haven’t checked for play in wheel bearings yet but will do asap. The brakes have 5k miles. Also flanges going into front diff feel tight and the diff is disengaged from 4wd. Any ideas?

Could both of these noises be related?

 

The first sound could be the rear axle front pinion bearing. Actually, both could be.

 

I thought of that as well, I figured if that bearing was bad it would get hot? I’ve felt the diff near the flange multiple times after a long drive and it’s warm to the touch. Normal. Also no play. Is there other ways to test it other then pulling the seal, flange and shaft to visually inspect it?

 

I would rotate the tires for the second noise. Might give a clue or help.

 

tires were rotated 5k miles ago I should’ve of mentioned that. I’m pretty sure it’s a mix of both. Cupping tires and wheel bearings are headed out. I’ll get to them but just curious if there’s a way both noises are connected.