What would cause a knock sound in the dash after turning off the AC-heat?

The knocking sound could be the doors of the vents closing when you turn it off,they are controlled by vacume.Or it could be the plastic making the sound due to change in temperature. If it is a early to mid 90's Chevy it is most likely the actuator solenoid that controls the recirc or fresh air. If it is it will knock for about a minute and 30 seconds when you start the vehicle and about the same wheen you turn it off.