There's a noise - a prominent low-pitched click/rattle - coming from somewhere in the front when 4HI or 4LO is engaged. It does not matter whether the hubs are locked or free. It starts at about 7mph and gets louder and faster with vehicle speed. It's louder going forward than in reverse. I jogged alongside the truck while someone else drove, and I'm about 90% certain it's coming from the axle, not the transfer case. The front driveshaft has no play in it. The whole front driveline has a good bit of play, but here's why I'm stumped: I just removed both birfs and the diff (took me about 45 minutes, never having done it before; I'm stoked with how easy this thing is to wrench on) and I can't find anything wrong, or anything very loose, either. Diff oil was opaque and goopy, but no metal in it. The ring and pinion gears have no play I can detect with my hands. Neither do the spider gears, though there is some axial (front-back) play in the side gears, less than 0.5mm (spec is 0.05-0.2). The pass. side axle has a small amount of play, the driver's side a tiny amount. How tight are the axle shafts supposed to be? Any chance I might be mislead and the noise is in fact the t-case? Any other ideas?

 

Wheel bearings?

 

Remove the front dshaft and see if it goes away.

:wabbit2:

 

Well, I had it up to 75 or so in 2wd on the 2-hour drive home, and heard nothing out of the ordinary from the driveline. It's only in 4wd and pretty loud.


The idea being that if the front DS is removed and there's still a noise, it must be the transfer case?

I just had it running in 4hi with both axles up on jackstands and the front driveshaft disconnected. No noises, but the load on the driveline was nearly nonexistent so I'm not convinced of anything yet.

I also discovered an electrical plug that was dangling too low and hitting the DS flange, but I have a hard time believing it could've been so loud. Besides, in 2hi with the hubs locked, the front DS is turning all the same, so I don't think that's it...

 

Hm. I'm reassembling both hubs, and my 85 FSM mentions a "thrust washer" that fits between the outer wheel bearing and the lock nut, inboard of the lock plate. Neither hub on my truck has this part. Is this part supposed to be present on an 81? What does it do, would its absence cause problems like what I'm encountering?

 

Yep. Outer wheel bearing. Washer. Has one tooth on it so it slides up groove in spindle. 54mm nut. Star washer. 54mm nut.

:wabbit2:

 

Well, I reassembled the whole works anyway, minus the parts I didn't have. Tested in the air (all wheels off the ground, 4hi engaged) after every major assembly step, no noises. Took it on the road, the noise is back, only in 4wd with the hubs locked. Happens regardless of acceleration or coasting, and it's a bonk that happens once per driveshaft rotation. So I don't think it's the axleshafts or hubs.

But I'm not sure about the diff. Because I'm upgrading the trans, case, and front driveshaft pretty soon, if the noise is coming from those parts, that's ok. But since I plan on keeping the front diff as it is, I want to investigate it in more detail. Any chance that a worn diff pinion bearing could bonk like this? I'm used to bad bearings making a screech, howl, or grind, not a slower rhythmic noise like this one.

I noticed that the driveshaft has, eyeballing it, ~5mm of runout. Is that normal?