C1500 vent system knocking

When I start the vehicle, or turn it off, I get a knocking sound inside the cab from behind the recirc vent door. It lasts for about 30 seconds and then stops. Also, if I turn the recirc system off or on, the knocking again occurs and then stops after about 30 seconds. The vent system seems to be functioning properly, but that knocking is really annoying. My bro said he had the same problem on his C1500 and unplugged something to make it stop. Any ideas on this strange problem?

I unplugged a module that is on the firewall above the recirc door. Problem solved. The recirc doesn't work now, but at least that annoying knocking is gone.