If this is the wrong forum just tell me, new guy and I'm trying to figure out where to start on a non-obvious problem. I'm somewhat skilled - can replace bolt-ons like a starter or fan clutch etc.

Ride: 1991 P/U, 3.0 VZE. 4" Skyjacker suspension lift. Auto hubs. 4.88 Nitros. Detroit on rear diff only.

Symptoms: Unidentified front end noise (I know this is hard online, but I'm trying my best). Discontinuous. Not so loud you hear anything different with the window rolled down. Truck starts and drives for some indeterminate period of time (minutes - 2 to 10), then a noise that sounds like some kind of rubbing (?) or howl starts. Sounds like it's under driver side foot well, but hard to be sure. It might cut out after a while or continue - lately it continues. There is a weak vibration through the floorboards. Engaging 4WD will make it stop. Slowing to a stop makes it stop - often once speed drops below 15 mph. Sometimes on engaging to 4HI, the pitch will change higher and it will get louder for about 2-3 sec before stopping. It cuts in and out abruptly. Engine temp and ambient temp make no difference. Frequency is tied to speed of vehicle - not revs. Loudest between 30-40 mph, but I think going faster road noise just drowns it out - it doesn't really go away. Turning doesn't make a difference one way or another. Some times will cut in or out when hitting small bumps at speed like expansion joints in a bridge.

Other:
Truck runs great, no performance issues in terms of engine, electrical, clutch, brake, handling.
Turns fine. No shaking through steering wheel. No shaking or strong vibration through gear shift or transfer case.
No issues in 4WD or transfering. No issues shifting. No lurching or other weirdness.

For the life of me I don't know what this is - but I'm hoping someone will tell me their best guess- because things like noises and oil leaks can go on for a while, then you find out you're in for a major repair. If I didn't know better I'd think something is either not disengaging fully or is loose. Obviously the tension caused by engaging 4WD does something - damned if I know what.

Talked with a couple buddies, the possibilities I got were:
- transmission
- CV joint

I've had bad CV joints - they seem to be accompanied by shaking. Just at a loss.

 

OK. I'm going to throw some more thoughts out there in the hopes that someone with more knowledge than me chimes in.

After doing a couple more things these are additional observations:
- putting transmission in N while driving doesn't abate the noise.
- if I put transmission in N and put TCase from 2HI to 4HI noise abates

I'm going to climb under it and make sure the driveline is tight on both ends. But - what do any of you think about the possibility that the output bearing on the tcase is bad?

 

the auto hubs might not be functioning correctly. when in 2wd, the hubs should disengage and the front stub axles should not rotate. the caveat is you need to go in the opposite direction from what you were traveling after using 4wd, ie, back up about 5 to 10 feet after shifting from 4wd to 2wd, then proceed normally. this should fully disengage the front autohubs, and the stubs shouldn't rotate. if they are rotating in 2wd, and you've made the "opposite" move to disengage, something in the hub might be broken or sticking. i get the same howl when i leave my manual aisin hubs locked, but travel in 2wd.

and this is not the correct forum. you are in the "build" sub-forum for '86 to '95 trucks/4runners. this should be in the '86 to '95 truck/4runner tech forum.

 

Not sure auto hubs were an option late as '91.

The Op is likely refering to ADD

This says auto hubs to ;89;;;; Replacing Toyota Auto Hubs: Off-Road.com (off-road.com)

 

Thank you . Can I re-post on that board? Just trying to show manners...

 

Yes. ADD. Sorry.

 

disconnect your speedo cable from the t-case and see if the sound goes away.

 

yeah, i glossed right over that. noted the OP’s statement of auto hubs, instead of ADD, and didn’t pay enough attention to the year.

 

Just throwing out options: Could do a front diff/trans/t case fluid change to see if that helps. How many miles on the truck?

 

I had the same type of issue on my truck and this is what it turned out to be. This should be the first troubleshooting step, because it takes only a minute to do and most of our cables and speedometer heads are pretty much at end of life by now.

 

I suggested disconnecting the speedo cable because the original post says:
Discontinuous
Sounds like it's under driver side foot well
some kind of rubbing (?) or howl
There is a weak vibration through the floorboards
Slowing to a stop makes it stop - often once speed drops below 15 mph
Frequency is tied to speed of vehicle
Turning doesn't make a difference one way or another

Every one of the above statements can be attributed to a bad speedo cable, just humor me, disconnect it and go for a drive. it will take 2 minutes to disconnect.

 

Truth brother! I rebuilt my locking hubs before I tracked the noise to the speedometer system. I have a thread going on it, and its still not fully resolved. To the OP, it sounded like a nest of agitated hornets or something rubbing on a tire and would cut in and out intermittently.

 

-Speedo cable
-brake component rubbing
-wheel bearings
-front drive train

I’ve been fooled on noises under rig before.

put a make on front driveshaft that is easy to see from outside of rig. Check it after driving. It should be in same place.