Blazer Loud Humming Noise

My 1996 Blazer made a strange noise yesterday as I was pulling out of my parking lot on the way home from work. As I came to a stop, this really loud humming noise came about and didn't stop until I started driving again. It was fine all the way home.

This morning it made the noise again, only this time as I was moving. I was decelerating to get off the freeway ramp and the noise got louder as I came to a stop. It then did this the rest of the way to work on and off as I stopped for several traffic lights.

I took it for a ride at lunch today and it was fine! I have no idea what is going on here!!! The only thing I've been doing differently is switching on the 4 wheel drive periodically on the snowy back roads and turning it off on the highways but I don't know if this is causing my problem or not. Any advice would be greatly appreciated.

Your 4wd might not be engaing/disengaging correctly. If you're going too fast and switching into or out of 4wd it will hum before it changes (that is if you actually slow down enough for it to change).

Thanks for the reply. What is the optimal speed to be switching in to and out of 4 wheel drive? The owner's manual does not specify a speed, so I assumed that it can be switched in any speed.

I would get your transfer case fluids and your front and rear differential fluids changed and your 4x4 system checked out.

It does indeed sound like the 4x4 system is not disengaging.

Yeah mine used to hum if I was changing at a speed over 15mph, but I had the transfer case and front differential fluid changed and it hasn't done it since.

I think we're supposed to able to "shift on the fly" at any speed, but I don't know if that's always good thing to do...

Yeah, on the way home yesterday, it was fine. I didn't put it into 4 at all though, so that would lead me to believe it has everything to do with the 4-wheel drive.

Thanks to both of you for your help. I'll get that stuff checked out.