hey guys i have a 92 gmc c3500 dually, it recently started making a clicking noise for about 10-15 seconds in the heater/ac box area under the dash when the key is turned on or off. it will also sometimes make the noise when the heater or a/c is running. i remember this happening on my dad's 91 gmc but can't remember what causes it. anyone know?
thanks,
chris

It sounds like what you are describing is a bad actuator door motor. These motors have a position sensor in them that loses connection, and then the system does not know the position of the door. When the position sensor fails the motor runs when the door is at the end of it travel and as the gears are jumping it makes clicking noise.

I see it’s your first post.
Welcome to the board.

thanks for the help, i'll have to check it out. the thing has been driving me nuts for almost a month now.
-chris