GM-GMC: a/c issues in 1993 one tone GMC, rough spot, safety measure

Question
the a/c is not operating.  the heater works fine.  the truck starts fine but there is a tapping noise coming from the passenger side under the front housing.  the tapping noise lasts about 30 seconds then stops.  It will sometimes come on when i am driving if i hit a rough spot in the road.  it also taps when i turn off the vehicle- again for 30 seconds or so.  and finally it taps when i try to turn on the a/c- which does not work- no cool air- hence i think the tapping is related to something in the a/c but if i knew what it was i could remove/fix/replace.
thanks


Answer
It sounds like you have one of the mode door actuators for the HVAC malfunctioning.  See if you can home in on the tapping noise to locate the door with the issue.  It is possible that the connection is loose or the wires are not on securely.  If it is the door for the heat/ a/c they generally go to the heat position when they fail as a safety measure (so you could defrost you windows).  It sounds like that could be the case with your truck, stuck on heat, even though the a/c system is working you can't get the air into the right place to cool things.
Hope this helps