Noise when decellerating

I have a 1995 blazer 4x4 with manual 4wd shift. Occasionally, I hear a noise when decellerating. It usually will start below 40 mph and continue until the vehicle starts. Most of the time, I can shift to 4wd-h and back to 2wd and the noise will stop for awhile. However, it always comes back given enough time.

Any ideas on what will cause this problem?

The noise sounds almost like what you hear on very knobby tires when a large truck is coming to a stop.

Thanks in advance!!

ED

Vacuum leak.