I have a 1999 Chevy Silverado 1500 4wd. When 4 wheel drive is engaged, there is a moaning sound from underneath the truck (can't tell if it's front, back, or middle). It sounds like gears moaning to me(but it doesn't make the sound continuously...ie...there are times while 4X4 is engaged that It doesn't make this noise). It does seem a little worse when I turn the wheels.

I've just recently replaced the front and rear diffs with new gear oil and checked the level in the transfer case (which was good). What could this be? The truck has 112,000 miles, but was previously owned by a golf pro for 10 years..pretty sure it was never put into 4X4 until I got it. Any help would be much appreciated!

 

Cycle it in and out about 20-30 times and run it around on a gravel road while in 4x4, drive it around on the gravel for about 10 miles or so each time. If it is still aking the noise after that, I would have to say it is probably the front diff binding up and putting pressure on the drive train. If that is the case than I would say that you need to check your tires, make surethat they have the same tire pressure and are the same height. If you have different style tires on the front verses the back but they are the same size, they may not necessarilybe the same overall height. Also tire pressure is a big factor. If the fronts are a little low then that could also cause the binding. If it is not the tires, then it will more than likely be the front diff.

 

Thanks! I appreciate it the help. I did notice that the left front tire was a little low yesterday. Also, the front diff has a small fluid leak (where the drive shaft attaches). When I bought the truck, I drained the front diff oil and noticed a fair amount of metal on the drain plug magnet...think this is normal wear and tear or could the previous owner have damaged it by driving it with low fluid? Also, I know this is a dumb questions (not all that familiar with how 4x4 works), but it he front diff always turning or only when in 4X4?

 

Yes, the front diff and front drive shaft always turn since they don'tuse lock-in-lock-out hubs anymore. When you are in 2 wheel the front output on the transfer case is not engaged (neutral essentially). When you put it in 4x4 the front output engages and turns the front driveshaft.

Personally, I wish they still used the lock-in-lock-out hubs.

 

Yes I agree, I wish they did too. If it is in 2x4, then the front drive line is free. It unlocks in the front diff and in the transfer case. (At least mine does) As far as the driving with low fluid, well that could be an issue, but I would only entertain that thought if I have exhuasted all other diagnostic routes! I would run it for a bit like I said and then see how it is. If it is still making the noise come back here and I will help you, unless someone else gets to it first!

OH, big point, if it sounds like it is getting worse, take it out of 4x4 and stop using the 4x4 until it can get looked at. If you get it locked up (REALLY BAD!!) then you will need to get a tow and go straight to a shop to get it fixed. So the risk is yours. Take it easy when out in 4x4 for a bit to see if it is going to get better or worse.

Have fun!! (Once it gets better!)

 

Does the front diff disengage from the drive shaft? I didn't think it did. I could be wrong though, I've never actually checked on this truck. I know in my old jeep the front shaft was always turning.

 

I've never looked, but I would guess that it does NOT disengage...As I always see a little gear oil leaking where the driveshaft enters the diff (even after driving in 2X4). I assume the driveshaft is always turning and the fluid leak is around the pinion seal?

 

Yes, the pinion seal could be leaking, that is pretty common on all differentials. Hell I think that my rear diff pinion seal is even leaking!