In my 1998 Chevrolet Blazer 4WD, there's a humming sound coming from the front sidewhen I take my foot off the gas pedal; it stops when I lightly press the gas pedal.

 

Front differential probably. is it in 4wd?

 

Thanks fr yr reply,
Noise comes in 2WD and also in 4WD. It begins when I take my foot off the gas pedal; it stops when I lightly press the gas pedal again.
We have checked the differential oil level: it's OK. Spinned the front wheels by hand: no grinding noise.

 

Now jack up the passenger front wheel (only), start the vehicle, and select 2WD. If 2WD indicator is lit steady, try to spin this wheel by hand. It should absolutely freewheel. Report please. Now select 4WD. Is indicator lit steady. Does front axle lock in and wheel not freewheel any longer? Also advise if you have 3 button or 4 button (auto) controls?

Ps I have had bad front u-joint make humming noises in 2WD but was steadier and at highway speeds

 

Many thanks fr yr reply and help.

I have 3 button contros on my car. I did what you said. When I lift the left side, the wheel spins in 2WD mode but not in 4WD. I did the same test to the right side, and it's OK too.

I noticed today that when I start driving in the morning, there is no the humming sound for the first 3-4 miles, then it starts again. The weather is gotten colder here, maybe it's not happening if the oil in the differential is thick.

By the way, I also want to mention that we didn't used the car for the last 5-6 months due to an engine overhaul. This humming sound wasn't there before; it appeared when we started driving the car again. Could there be some rust on the differential gears?
Thanks again