2002 Chevy Avalanche Noise when all-wheel drive is engaged

Developed noise problem when driving in 4-wheel drive, and driving above 18 mph. At highway speed the noise sound turns into a whining sound (e.G. Table saw running), however, as soon as the 4wd is turned off and put into 2 wheel drive, it disappears. It just started happening, I tried parking it in the garage for the weekend, because it was extremely could in Anchorage the last 2 weeks while I was on vacation, but that didn't seem to help.

Samantha