98 expy

I have a hum/moan noise and a slight vibration when I'm in 4hi.

I only hear it on a clear(still wet) patch of road, when I hit a snowy patch, the noise/vibration is gone.

My last truck was a AWD explorer, so im not that familiar with "4x4" systems.

 

In 4hi the transfer case is locked front to back. Any difference in the size of the tires and when turning it causes strain on the drive-train until either the front or rear wheels slip. I have AWD mode but on my truck where its just 4-hi I try to only use 4-hi when its mostly snow covered.

 

My tires are matching sizes. I only use 4h on snow covered roads but sometimes there is a stretch of road with less than a inch and the noise occurs.

The noise is similar to a bad wheel bearing but only happens in 4hi.

 

Small noises & vibrations are fairly normal when you are in 4 wheel drive especially on older trucks. I would think if something is wrong, you would hear the noise even when you are in snow. Maybe an alignment problem giving the vibration when the transfer cases are locked.

 

Even with the same tire sizes, if one axles tires are slightly more worn than the others it could be putting a little bind on the drivetrain... Nothing you should worry about but a good reason to stay out of 4x4 unless you need it. Snow allows for a bit of slip so you dont have any bind.

As a general rule of thumb, I only use 4x4 to get unstuck or perhaps to avoid getting stuck. I think they say to keep it under 35 mph to avoid to much wear. Anything faster really doesn't need 4x4. I submit that anything over 10 doesnt if your not slipping.

I am sure different people will have their own take on this... For me, occasionally when I go out to have fun in the truck, I find 2wd is way more fun cause your steering left to go right.

 

Are your front and back tires the same brand they very in size despite the markings? I've even experienced this with two new tiers on one end and used on the other.

 

my '98 has been locked in 4x4 for months and is every winter with no odd noises or vibrations with speeds up to 70/75 mph on the highway.

some trucks are a little different unless its a pronounced noise i would not worry.