[Chevyaz]

Can someone please tell me what the hell is a flapping sound coming from the blower motor area on my truck. It makes the sound when I start the truck and when I turn it off. And after I turn off the heater. The sound doesn't last long maybe about 15 seconds or so. My heat works fine and the ac works also. Its a 1994 gmc silverado truck 350 5.7 2wd.