Horrible Clicking Sound in Air System

Ok its a Chevy.... but it please help..... its a '96 Chevy Tahoe and on the right fan area in the inside around the glove box when you start the truck up or its running it all of a sudden makes this sound like a fan hitting foam. And my uncle is a Mopar car guy looked and tore it apart and couldnt fix it. And if you shut the truck off it stops most of the time, some times not. And its very new and the truck doesnt have a ton of miles on it. And we think its possible a vacuum thing can you help!