I had never thought of that. Will the tail shaft assembly fling tranny fluid everywhere? If not, that's a great suggestion, and well worth a try.

 

Latest: I put the rear end on jack stands, removed tires, drums, both rear axles, and the differential gears (I left only the pinion bearing and drive shaft hooked up), then ran the truck in gear up to 70 mph. The resonance/vibration problem was still there, unchanged. That eliminates all the pesky "what-if's" that were bugging me (tires? faulty wheel bearing? Worn inner axle bearing? etc.), because none of that was spinning. This leaves only the tranny, transfer case, or drive shaft as potential sources.


My conclusion: I strongly suspect it's a transfer case problem. The noise is speed, not RPM or gear-dependent, which eliminates the tranny in my mind, and the drive shaft has all new u-joints on it, and only about 1mm of play/run out on the rear, which I assume to be well within specs -this eliminates the drive shaft as the source too. So... I end up with the transfer case as my prime suspect.

My next move is to drive it to a trusted tranny shop, drop the keys off, and say "fix it." I wrote down all of the above, mostly to conclude this thread in a way that could be helpful to others with similar problems in the future. As I looked around for help diagnosing this issue I didn't really find much, which is why I started the thread. Maybe my problem, research, and conclusion can help somebody else.

 

what in the transfer case is moving at the same time? Nothing except front wheel hub bearings.

 

I disconnected front drive-in as well, for good measure (didn't want any wheels having connection to motor while I ran it up to 70 mph on jackstands). So, front hubs etc. Weren't involved.

I don't know the names of all the interior transfer case parts parts, but the rear drive linkage must go through the transfer case (whether in 4x4 mode or not) then the tail shaft in order to deliver power to the rear drive shaft. I'm sure there are bearings, and other sorts of goodies in there. Those are all spinning at vehicle speed, rather than RPM of engine, and the culprit has to be something along those lines. Ultimately, my theory is irrelevant, its either something in the tc, or tyranny, and either one is a job I'd rather pay the shop to do.

 

Barnesam,

I found this forum and your question because I am experiencing the exact problem on my 1999 GMC Suburban. I had the u joints replaced, thinking that would fix it but it has not. I want to try driving it with the rear drive shaft removed and in 4wd, but I dont know if that is possible or if transfer case fluid will fly everywhere. I believe the problem with mine is the driveshaft. I am in the process of locating a junkyard driveshaft to swap out to see if that eliminates the problem. Let me know if you find a solution to your problem.

 

Barnesam,

The other thing I have not tried yet but will try this weekend is to remove the rear driveshaft, rotate it 180 degrees to the rear yoke and bolt it back down again. My resonation problem started as a result of replacing the u joints due to the loud clunking noises when putting it in gear. Thats why I think its a driveshaft problem. I took it back to the shop thinking they may have accidently knocked off one of the driveshaft weights, but they said it all looked good. They do not have a solution to my resonation problem either.

 

You know, I had suspected drive shaft too. My problem started around the same time I had the front u-joints in the rear shaft replaced. I've since replaced the other set as well (which ended up not being bad), then taking the shaft to a shop to check it out, and their opinion was that all the joints, weights, etc. looked just fine. Granted, they were not a drive-line shop, so they could have been wrong, but I also removed my rear differential gears and ran it up to 80 mph in gear on jackstands, and there was only about 1mm of runout in the drive-shaft. Just to test it, I then removed and rotated the shaft on the yoke like you suggested, and there was no change.

The truth is I've pretty much just parked the suburban for now. I haven't given up, but it's an extra vehicle for us, so my wife's just been driving her van, and I drive my commuter car, and the 'burb sits (we have 6 kids, so we only need it when we all want to go somewhere together, which we haven't done a lot of lately). I figure I'll take it into the transmission specialist next month sometime to have them give me an opinion.

I have thought about your solution too, of throwing a junkyard drive shaft on just to see how if the noise changes/disappears. If you do that, let me know what happens. I'll be very interested to hear.

 

To anybody who stumbles upon this thread looking for answers to a similar problem, here was the answer to mine: DRIVE SHAFT!!

I mentioned in my original post that two things had happened at the onset of the noise, one was the u-joints were changed in the drive shaft, the other was the wheel bearing that went bad. Well, as it tuns out, the shop who changed the u-joints for me BENT MY DRIVE SHAFT in the process.

I diagnosed the problem by removing the drive shaft, then running the truck in drive up to 80 MPH. Smooth as silk (no noise/vibration)!!! Took the drive shaft to a drive-line balance shop, and they said it had been bent bad enough that repair/rebalance was not possible. They build me a new one for $290. I got it home, installed it, road tested the truck and it rides as smooth and quiet as a Cadillac! It's now back to being fun to drive (king of the road)!

Problem/Solution: drive shaft bent by a careless mechanic; installed brand new drive shaft.

 

Hope you took it back to the shop that bent it for some renumeration. Or to just throw it thru the window.

 

barnesam,

The problem with my suburban was the driveshaft as well!!! After checking just about everything else possible (tires, rims axles, transfer case etc,) I pulled the driveshaft and took it to a shop to have it re-balanced and I found a sizeable dent at each end (probably where the shop that replaced the u-joints clamped it in a vice). I had to have a new rear drive shaft built, But the problem is gone!!