Hi....my 99 4w drive blazer is making a very loud whining noise at speeds under 40 MPH. The noise goes away over 40. Sounds as if it coming from the front of the truck but hard to be sure. I coasted the truck with it turned off and the noise went away, so I don't think it's wheel bearing related. Transmission shifts fine and engine runs well. Sounds like an electric motor that runs at high volumn then drops slowly in rpms. Good with mechanical issues but don't have much experience with four wheel drives.
Thanks

 

To rule out a bad belt pulley,pull off the belt and crank her up. Does she make the sound while at idle?

 

Thanks...will give it a shot tonight. Sound isn't there when idling.

 

see if its there idling,but in gear. maybe it only does it under load.

 

Doesn't occur when idling even when it's in gear. The noise starts out at a low rpm then revs up high and loud...then drops down to nothing when totally stopped. Like drilling through something with a hand drill.

 

Could it be the encoder motor?

 

Pulled the fuse to the encoder motor at lunch, noise gone. Must be jammed or transfer case problems.

 

clutch fan.

 

Stick the fuse back in for the encoder motor. Drive it til you hear the noise, then select 4WD HI and see if the noise changes. Don't leave it in 4WD, only long enough to see if the noise changes, (4WD should not be used on dry pavement).