2005 QC 4.7 4x4 195k miles

I am getting a fairly loud humming noise from the front end that starts and is loudest at about 30 mph and tapers off until it almost goes quiet over 40-45 mph. This only happens when the truck is in 4 Lock mode (haven't tried 4 Low, I don't go that fast in that mode anyway.) I only noticed it recently because we had some snow and I used the 4 wheel drive for the first time in months.

The noise can best be described as resonance hum coming from the drivetrain. The pitch does change with speed. No grinding, clicking, chattering or anything like that. It sounds like normal 4x4 noise, only amplified through that speed range.

I had the front diff cover off yesterday to change the oil, and all the gears look good. I need to change the transfer case fluid still; I couldn't get the hex nuts to break loose yet.

Since everything is always connected and I only get the noise while 4 wheel drive is engaged, is the transfer case the likely place the noise is coming from? What can I do to isolate the noise or determine if it's even really a problem?

 

Probably normal if you have not ever used it before. Check the diff and transfer case fluid levels, if ok, I wouldn't worry about it.

 

I forgot to mention that this is a new noise as of this year. The new noise sounds normal, just amplified, like the tone is resonating or something. I've had the truck since 2009, bought it with 53k on it. I don't use the 4 wheel drive much except for the winter.

I've got to figure out a way to get the nuts out of the transfer case to change the fluid in there. They are stuck pretty good.