Yeah, it's not a side to side vibration like a tire or a brake rotor. Like I said, it feels like maybe a stick is caught under the truck frame, dragging the ground, transmitting minor sensations and noise into the truck. It's not a shimmy or wobble or any major vibration, it's subtle. Like a bad wheel bearing, except it doesn't respond like a wheel bearing when you go around a curve...and a wheel bearing won't change depending on throttle load, but this does.



Having uhm, ahem "tested" the ABS in the snow a few times since I've owned this truck, I can say that it's most definitely not the ABS pump. It's more subtle than that. A driver less "in tune with" their vehicle (I've owned this truck for 5.5 years) might not even notice it, although they would notice the noise. IT's probably half the noise intensity of the ABS pump, with about 1/3 the vibration intensity of the ABS pump. And it only happens between 30 and 50mph at coast and light deceleration. That would describe it well.



No pulling whatsoever. Drives perfectly other than the noise and minor vibration sensation at coast and (sometimes) deceleration.

Yes, I have the np149 single speed awd tcase.

I hadn't considered the viscous coupler, thanks for the idea. Maybe I will get out the FSM and do some reading on it. I do know that with the rear wheels on the ground, it's impossible to turn the front driveshaft more than 1/8 of a turn, and very slowly at that, I know that because it made it difficult to put 2 of the 4 unjoint retainer bolts back on the front diff a couple days ago, they were pointing up toward the motor and I couldn't rotate the front shaft enough to gain good access to them.

I seem to remember seeing a test for trying to rotate the front shaft as a test for the viscous coupler, so I'll research that.

 

Try going to the denali trucks website, if you have never been there. They have a wealth of knowledge on that board when it comes to our trucks, a very nice archive too.

 

Yeah, I've been there before and registered a few years back to facilitate the purchase of my denali lower front grille insert from a guy in australia who makes them.

Anyway, today I've done more troubleshooting. I jacked the front of the truck up and put jack stands under the frame. I jacked the rear diff/axle up and put jack stands under the swaybar mounts on the axle. So all 4 wheels were in the air and able to spin free.

I started the truck and put it in gear and let it idle with all the wheels spinning, listening to each one as I walked around. The noise seemed to be coming from the drivers front area, not the wheel itself.

I got underneath and put my hand on the front diff, it was smooth and quiet. Same for the transfer case. I noticed no extra movement in either driveshaft.

I put a brick on the gas pedal to gently nudge the wheel speed up to the 40mph range, and promptly got the noise and vibration to return. Again I checked the front diff, quiet as a mouse. Transfer case was rumbling and vibrating significantly. Transmission did not seem to be the source of the issue, the noise seemed to be coming from the output shaft area.

I then removed the front driveshaft and repeated the test. Tcase is quiet as a mouse without the front shaft.

So I'm going to replace those front shaft ujoints since that is the cheapest and easiest thing to try right now.

I do see a balance weight on the shaft, but I don't see how that translates into balancing or matching up with either the tcase or front diff in terms of balance. But if the problem persists I may take the shaft to a shop and have it rebalanced.

Will update thread after I've installed the new joints.

 

I wonder if one of your front diff side bearing adjuster locks has come loose. Not an uncommon thing to happen on that model truck, and gives symptoms like you describe. If that's what it is, and you let it go for too long, you'll be doing a complete overhaul instead of a basic repair. It won't act the same when you're running it in the air, because there is no load on the ring & pinion.

 

Thanks for the reply. But, as stated in the first post, I've already disassembled and rebuilt the front diff per the factory service manual specs. I made certain to adjust the diff carrier bearing adjusters as instructed and bent the lock tabs down as firmly as possible with a hammer and a punch.

Since the problem was present both before and after the diff rebuild, then the problem doesn't seem to be related. However, I'm still glad I did that, because it had quite a bit of slop and worn bearings/metal shavings in it anyway.

 

Well, it seems that the new front shaft ujoints fixed the problem. There wasn't really anything wrong with them that I could tell, which is puzzling. They were about 3 years/50k old. One end of one joint had a couple of rollers partially broken off at the end on the inside (out of 20-30 rollers) but they still seemed to function normally.

I put in new joints and reinstalled the shaft exactly as it was before.

After I put the new joints in I left it jacked up and took it up to 40mph, smooth and quiet. Took it for a 10 minute road test, noise and vibration seem to be gone.

I still can't figure this **** out. But, at least it's fixed.

 

Man I hate troubleshooting noise NVH complaints they are a real bitch sometimes like what you went through. Glad you found it. It can literally be anything when you start having problems like yours.

 

I'm glad that you posted all of this because I have a similar problem with my 06 silverado. I have no steering issues but a grinding whine when I am coasting in a parking lot. Or slowing down.

 

I've noticed this same issue on mine. I'll have to get it checked out and see its the u-joints need to be replaced. I usually feel it between 40-50 mph.

 

I have the same problem on my 2000 Chevy Silverado 4x4 auto track transfer case. The only problem is I created it by installing a Mc Gaughlys 7 inch lift kit. They stated that the drive line angels remain at stock specs with there lift kit but it is not true.Well any way back to your truck the problem is the drive shaft angel. The angels changes when you let off the gas or accelerate. This may be due to worn out u joints, But in your case you replaced them. So the last thing that that is causing your drive line angels to change when you accelerate or decelerate is your motor mounts and transmission mounts. They are probably weak and shift slightly when on the throttel. I Would recomend GM OEM parts they will bring back your drive line angels back to specs. On my truck the combination of a 7 inch lift kit and 125k on the worn engine mounts are causing this drive line drone noise/vibration. I have not had the time or money to fix my truck so what I did is removed the front drive shaft. You can drive it with the drive shaft removed as I can vouch for that because that is how I am driving my truck every day 50 miles round trip to work. Just make sure you cover up the transfer case output shaft hole with duct tape so dirt and water do not get into the output shaft hole. ASE Master certified mecanic for 15 years and GM dealership experiance 14 years. Ray T.