"auto" 4wd Noise

zedbear · November 11, 2009

Been chasing some phantom front end noises on this truck, like a low humming type noise. Put it in Auto 4wd to see if it would change anything and it got a bit louder and I could feel it through the floorboard a little. This humming sound/feel seems to be most noticable at 45+ mph. Subsides a bit when I put it back in 2wd. I then noticed some rubbing sounds or a rough feeling from the front end when turning a corner at low speeds in Auto 4wd. Doesn't do it when it's in 2wd.

What could be the problem? The bottom of the front diff is oily but I'm not sure if it's just p/s fluid and motor oil from an unrelated issue. Could it just be low or worn out diff fluid or something more sinister? Thanks.

C. Charles Hahn · November 11, 2009

How do the CV boots look on the front axles? It almost sounds like you could have a bad CV joint or you could have a failed pinion seal at the front differential.... just a guess. Obviously check the fluid level and condition first.

I'd clean everything up at the front of the truck with brake parts cleaner and then see if the fluids come back. If they do, it should be easier to identify the source.

Matt_ · November 11, 2009

Drain your front diff fluid and refill. Take a look at the fluid to see if there are any metal particles. A humming sound i usually due to a bearing thats worn or gear noise from lack of lubrication. A rough feeling at slow speeds and turning when in "auto" makes me think a bearing might be going. A worn bearing adds resistance, so when the front axle shafts are engaged it will almost feel like they're in 4wd.

From what you described, i don't think it's the CVs. Bad CV's dont make a humming sound. Also, they are always spinning... so if you had a CV going bad, it wouldn't go away when you put the truck into 2wd.

While in 2WD take off the front driveshaft and see if you can spin the pinion by hand. Also make sure there's no play in it. If it "grinds" at all when you turn it and/or it feels loose, you need some diff work.

Stevens03 · November 11, 2009

+1 on what matt said..

Crazy Canuck · November 11, 2009

Been chasing some phantom front end noises on this truck, like a low humming type noise. Put it in Auto 4wd to see if it would change anything and it got a bit louder and I could feel it through the floorboard a little. This humming sound/feel seems to be most noticable at 45+ mph. Subsides a bit when I put it back in 2wd. I then noticed some rubbing sounds or a rough feeling from the front end when turning a corner at low speeds in Auto 4wd. Doesn't do it when it's in 2wd.

What could be the problem? The bottom of the front diff is oily but I'm not sure if it's just p/s fluid and motor oil from an unrelated issue. Could it just be low or worn out diff fluid or something more sinister? Thanks.

My mom has an Envoy and for whatever reason I was driving it last fall and noticed the same thing you are describing, although there seemed to be little or no noise in 2wd. Turned out to be the bearing in the transfer case. I would tend to think it might be that as the diff, like the CV's spin all the time, and you seem to have an issue that is amplified once the transfer case engages the front driveline.

Matt_ · November 11, 2009

Greg, in his front diff only the spider gears spin all of the time. When in 2WD the carrier doesn't spin, especially if a bearing is going bad since it resists rotation. So the vibration would go away in 2WD mode whereas the CV's issue wouldn't.

However, you are right... t-case issues could cause the same symptons. Anything that adds pre-load to the front driveshaft will cause a "binding" feeling. This could be a bad output shaft bearing or a sign that his encoder motor is putting too much pre-load on the clutch packs in Auto-mode. Some trucks actually will always experience binding in Auto-mode because of the clutch pre-loads, and this is normal. But it sounds like this is a recent issue for the OP.... and with the front diff potentially leaking, thats a good place to start. (edit: increased loads from the clutch likely wouldn't result in noise like the OP is describing... probably a bearing somewhere)

zedbear · November 11, 2009

Yeah, I've been wanting to change out both the diff and tcase fluids. I forgot to mention that I put it in 4wd high briefly (like 10 secs) on a staight and level road the other day and the humming got much louder, especially if I accelerated. I do still hear "something" in 2wd but it's not noticeable till I get up around 45mph or more.

Any chance this could be a front hub issue as well?

Also, should I get the needed fluids from the dealer? Thanks.

Spurshot · November 11, 2009

I chased a significant rumble in my driveline last fall.

1) I'd recommend you jack the rear tires off the ground and put on stands (pull the wheels if it makes you more comfortable), chock the front tires, place the truck in 2 wheel drive, start the truck and run it to the speed range you believe should cause the noise.

2) once you are sure the noise is present, shut the truck down, remove the front driveshaft, repeat the test above.

Keep in mind, the front driveshaft turns even in 2 wd due to the drag of the clutches in the transfer case. You can turn the front driveshaft by hand while in 2 wd but should feel some drag.

My problem turned out to be the front driveshaft itself. I had already changed the u-joints and in chasing the rumble, I had the driveshaft balanced, checked for straightness, etc. The transfer case was even rebuilt and the output cluster with the spline was replaced in an attempt to remedy the problem. The TC rebuild did reduce the rumble by about 60-70%, to a more acceptable level, but it's still there a bit. The front differential was also rebuilt in pursuit of the remedy. I have purchased the spline yoke for the front driveshaft and haven't installed it yet. It's the only part that hasn't been changed on the truck. None of the above may apply to your truck, but I thought I'd share my problems in case it helps.

After an exhaustive search on the net, I found one story where a guy had bought a truck used from a dealer with a similar rumble detected prior to the purchase. The dealer promised to resolve it if the truck was purchased. The sale occurred and the buyer returned the truck for repair. The dealer changed u-joints, rebuilt the transfercase, front and rear diffs, replaced shocks (?), rebuilt transmission, all to no resolution. The dealer finally replaced the front driveshaft with a new GM assembly. That cured it.

BTW, my rumble started around 45 mph and got progressively worse with the really bad, mirror vibrating, rumble at about 75 mph.

zedbear · November 13, 2009

I may try that but it seems a little sketchy, lol. (has images in head of truck crashing through the garage wall ).

I went to the dealer today and asked for some front axle lube and tranfer case lube. He gave me two 8 dollar quarts of "Autotrack II" for the tcase but informed me the front axle lube is 38 dollars a quart!! Thats just ridiculous. He told me to go to Napa and ask for 75w-90 synthetic for the front and the rear diffs. Is this OK? I hope so because I'm not spending 80 bucks for 2 quarts of magic fluid, lol.

Matt_ · November 13, 2009

Check your owners manual.... for your 2000 model year I believe you need 80w-90 NON-SYNTHETIC for the front. The seals on the front diff have been known to break down and leak if synthetic fluid is used.

zedbear · November 14, 2009

Check your owners manual.... for your 2000 model year I believe you need 80w-90 NON-SYNTHETIC for the front. The seals on the front diff have been known to break down and leak if synthetic fluid is used.

Hmm. Parts guy at the dealer checked and said they use synthetic 75w-90 for everything, front and rear. Whether he knows what he's talking about, I don't know....

I changed the t-case fluid today. Stuff that was in there looked brand new still, a nice green color. When I pulled the fill plug prior to draining, a bunch starting flowing out. Is this normal or was it overfilled prior? It was warm still so perhaps it was just from expansion? Same thing happened when I pulled the front diff plug to check fluid condition. That fluid was much darker and dirtier looking but still clear enough to see though it.

zedbear · November 17, 2009

Humming noise seems more pronounced now. Still sounds suspiciously like a front wheel bearing and seems to be getting more noticable even in 2wd. But I just changed this t-case fluid and it seemed to get worse in the days after that. Coincidence? I hope so.....

Still gotta change that front diff fluid.

jsklenar · November 17, 2009

First, when in auto the transfer case is not engaged. Auto locks the front diff. only so that rules out the transfer case.

Second, check the front axle seals. it is very common for these to leak. They are around $18 for the seal. Drain the diff fix the leak and refill with the corrct fluid.

Third it could be a hub bearing. Also common on these. If you drive down the road and steer from left to right repetedly see if the noise goes away in one direction. If it does it is a hub bearing. Also jack up the front end and move the tire top to bottom and side to side to check for play. Though these bearing often make noise with little to no play. the bearing will come as a hub assembly.

TYPE GENDY · November 29, 2009

I remember having a similar issue last winter when i was in auto, gonna have to try and track the noise down and hopefully get my fluid changed before the winter is here if thats the problem..

Matt_ · November 29, 2009

First, when in auto the transfer case is not engaged. Auto locks the front diff. only so that rules out the transfer case.
Second, check the front axle seals. it is very common for these to leak. They are around $18 for the seal. Drain the diff fix the leak and refill with the corrct fluid.

Third it could be a hub bearing. Also common on these. If you drive down the road and steer from left to right repetedly see if the noise goes away in one direction. If it does it is a hub bearing. Also jack up the front end and move the tire top to bottom and side to side to check for play. Though these bearing often make noise with little to no play. the bearing will come as a hub assembly.

Not quite. When in auto mode there is pre-load added to the clutch packs. Usually this is not enough to transfer any power to the front, but in some cases it does. This is why some truck get some binding in parking lots when in auto mode and other dont. Depending on how much pre-load his truck has, you can't fully rule out the t-case. Also, his encoder motor sensor could also be going out of cal and causing it to apply more force than it should.

Hub bearings will be an issue in 2wd, 4wd, auto, etc.. If the noise goes away when he puts the truck in 2wd, its not likely a hub bearing.