New here, and appreciate any help.

Truck is a 2001 4.7 4WD. 187K. Done quite a bit over the last few years, and she's run pretty consistently well.
Recently, however, I heard a low rumble coming from the rear end that sounded like wheel bearings to me, and began to get gradually louder. I eventually pulled the axles and replaced both axle bearings and seals with Timkins. Axles looked fine, so didn't bother with the "axle repair" bearings that are available. Drums also looked fine. Put it all back together, filled up the diff, and the sound was still there. Now, almost 2 weeks later, seems like the sound is getting louder to me. It is very noticeable; like a low grumbling/whirring that follows wheel speed, regardless of acceleration or gear. Increases in pitch with speed (obviously). I jacked up the rear, spun the wheels- can clearly hear the noise. Same running it in neutral on stands. I got under it, put my ear to the diff, and had someone else spin the wheels- heard nothing (near the diff). Noise is crystal clear though, even when in neutral.

Here's my question; if it was the pinion bearing all along, wouldn't the sound be coming from near the diff/driveshaft? Not the wheels? Also, as the driveshaft is not my specialty, one other question. When I jacked up the rear, then started the truck in neutral, the rpms went up quite a bit from their normal idle range, and the drive wheel began spinning quite fast (like way too fast to stop with your hand). I know an automatic in neutral is still applying a small amount of torque to the driveshaft, and I don't want to sound dumb, but is that much normal?

Any help would be appreciated.

Thanks!

 

If only one tire is spinning, it's gonna be going twice as fast as when the truck is on the ground, and both tires MUST turn.

There shouldn't be a whole lotta power behind it though. Stepping lightly on the brake pedal SHOULD stop it pretty quick.

Bearing noise can be weird... especially when it has all those lovely steel tubes/shafts to transmit it throughout the truck. Hold a screwdriver against your ear, and press the other end against various spots on the rear diff. Should be able to isolate the noise that way pretty easy. Just be CAREFUL of spinning parts.

 

What HeyYou said. Using a 12" screwdriver as a stethoscope is an old mechanics trick. Same with a piece of 3/8 or 1/2" rubber hose.

And yes, Neutral still applies engine RPM to the driveshaft. With hydraulic pressure removed, it's parasitic drag coupling all 3 sets of planetaries. Tapping the brakes or setting the parking brake will hold it back, or at least to a managable level.

Being an 01, pulling the driveshaft is quite simple actually. It's four bolts mating the flanges, IIRC 17mm wrench. Apply copious blue loctite when reassembling.

Also don't overlook that it could be the differential carrier bearings inside the case. Quite unlikely as they don't see the dynamic loads that the wheel bearings see, but I've seen it before, especially if it's been abused in 4WD mode.

If you pull the driveshaft flange off, something you can do is get a socket that fits the pinion nut, use the proper adapter(s) to chuck it to an electric drill (battery will work, 120 electric preferred), and spin the pinion with the drill. Milwaukee M18 is rated 1800 RPM, about 40 MPH on the speedometer. Dewalt electric is rated 2500 RPM, about 55 RPM. Bonus points if you also take the tires off.

Then you can verify that the noise is in the rear axle and not the transmission tailshaft or a U-joint. It also removes a couple harmonic noise sources from the driveline so you can better identify where the noise is coming again either by ear or improvised stethoscope.

 

I have an actual stethoscope, and tried listening all over; with the car running; while spinning the tires; under the diff while someone else spun the tires; with the car in drive (sketchy, I know).
very hard to tell with the car running, since there’s so many moving parts along the driveline; when spinning the tires though, really doesn’t sound like it’s coming from the diff. The axle could be tricking me, but sounds so clearly like it’s coming from the wheels.

 

Ok, pulled the driveshaft as recommended. Unfortunately I don’t seem to have anything powerful enough to spin the pinion though (my electric impact will just start to but as soon as the impact kicks in it drowns out all other noise lol).
I tried moving the pinion by hand though, and it spins easy enough; also no up/down/side to side play- assuming there would be either play, or chunky movement if the bearing was done for?

Like I said to HeyYou, I listened all over with a stethoscope and still sounds clearest at the wheels (altho the sound def travel at least a ways down the axle shaft).
I've added a couple videos trying to show what it sounds like- still screams bearing to me.
Is it possible the gear oil isn’t getting all the way to the axle bearings for some reason (can’t imagine why)?
Any other thoughts?

Oh, I should also note when I drained the diff, there was def substantially less gear oil in there then the manual calls for; no visible leaks, and def still a decent amount of oil (maybe 1.5-2qt). I suppose that could have damaged something, but nothing obvious when I had it open.

Appreciate the input very much!

 

Ok, site wouldn't let me upload the videos directly for some reason, so here's the Youtube links:
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Did you install the bearings dry? It takes a bit for fluid to make it's way down there... and I have smoked brand new bearings that way.......

 

I brushed them with a little gear oil, but no grease

 

I tend to soak 'em, then install. It does sound like bead bearings though....... Hate to suggest it... but, pull the axles again, pull the bearings, and see what they feel like.

 

That’s what I was afraid you were going to suggest lol.
I think you’re right though- no other way to know for sure. I’ll try to pick up some bearings/seals in town.

May not get to it for a few days- assuming I should probably avoid driving it in the meantime? I know it’s unlikely anything catastrophic would happen, but hate to risk damaging the axles.

Thanks again