Hello,

I have a 1996 4 door blazer. As the title states I hear a humming noise with a very slight vibration, which can be felt the most on the passenger side floor board. I checked the hubs and they are fine.

Here is something interesting, when i engage 4wd the humming/vibration goes about 98% away, and when back to 2wd you hear it start up again.

I replaced the little vacum switch on the transfer case and that didn't fix the problem. You can also feel it in the driver's floor too, which also leads me to believe it is something with the transfercase. I don't think it has anything to do with the u-joints, since it is very smooth when driving and novibration at all on the steering wheel. Ita vibration/hummingin the floor.

Any help would be greatly appreciated.

Thanks very much.

 

I would check the transfer case and rear diff oil.
You may have to jack the rear wheels off the ground, put the truck on jack stands and run it to help with the diagnosis.
When you feel a vibration in the floor it's usually from the rear of the vehicle. Vibrations from the front are felt in the steering wheel.

 

Thanks for the suggestions.

Today driving it, rather than a humming noise, it sounds more like awining noise combination. Last year I checked the transfercase oil and it was bone dry, so i filled it back up and the noise still the same.I did purchase a used one, but never got around yet to putting it in.

I still find it very intersting that when I put the truck in 4 wheel drive, the noise almost completely goes away.

The truck is extremely smooth riding except for the vibration in the floor board in driving and especially passenger floor board.

Any other thoughts???

Thanks again.

 

Hello,
It has been my experence that whenthe front dif is allowed to becomemore than 1/2 lowonlube, it willcause excessive wear onthe spider gears,and if driven alot with the condition present,it will alsotake out the carrier, pilot bushing etc.

G.M. has a tsb that describes the very condition you explain.
I have personally just replaced my new to me,'05 spiders(lucky under warranty)
It corrected the "whine/growl condition in 2wd only.
Some yearsago, Ihad to replacea set of spider gearsfor a customer in a'94I had in for service,
It had gotten so bad ,that whenit wasput it in 4x4 and accelerated hard, it will "shudder" and act like the front tires were agressive treadorthe CV joints were bad.

The front spiders by design,turn all the time it is in 2wd. They need to be bathed in lube, unlike a rear dif that is lubed by the ring/pinionscontinious rotation.
I hope this helps

 

Hey, thanks very much for your feedback, I really appreciate it.

Any thoughts on why when I put it into 4wd the noise certainly is reduced??

Is there any way that I can check the front differential to check the gears??

Thanks very much,
Chris

 

Hi chris,
When they are in 4x4,the whole spider assy is rotating together so the spiders are not turning individually. as the whole ring/pinion carrier is turning, the spiders are quiet.
There is no way to inspect the dif except to pull it and take it apart to inspect.
vibration usually indicates a out of balance condition likeU joints in the rear drive shaft.
I did experance a noisy/vibration condition on a high milage 1st gen.--turned out to be rusted body mounts allowing the body to settle on the transfer case and normal noise was transmitted up through the floorboards. causing alot of noise.
If it is the front spiders,they should be completly quiet in 4x4.
You say the T case was bone dry,-- that was sure bad for the bearings and associated gear/chain.
have you checked the front dif for lube?
Possibly you have mutiple things going on here.
I get uneasy knowing you had a dry t case.
If it were me,I would change it as you already have one, and check the U joints then go from there if the problem still persists.
luck!