Debra Bellingham, Washington - 1994 GMC Sierra 1500
Debra’s hearing a mysterious knocking sound–it happens at random times, sometimes when the AC is on, sometimes before she turns the key, and sometimes after she turns the key off. It seems to be coming from behind or below the glove compartment–there’s a little box below the glove compartment. Tom and Ray are fairly stumped–it may be something caught in a fan, like a twig. It sounds like a power antenna motor, except the car doesn’t have one!
Come on, guys, I know you’ve answered a similar question on a different vehicle before. This is the air diverter door for the AC. It’s probably controlled by an electric motor, and the limit switch is bad. It’s made to default to the defrost position when you turn off the key. When it moves to the defrost position it’s supposed to stop when it hits the limit switch. If that switch is bad, it will keep driving the motor and a ratchet or knocking sound is made until it times out. A twig or pine needle striking the fan blades sounds totally different and would vary in frequency with the blower speed.