Auto Air Conditioning & Heater Repair: knocking noise inAC/Heater system, thump thump, 2007 chevy impala

Question
we have a 2007 chevy impala. within the last 2 weeks we have started to hear a knocking noise under the dash.  it seems to come from the ventilation system.  i don't know if it is a gate that is stuck or a solenoid trying to open and/or close.  it lasts about 1-2 minutes and stops.  at this time it does not seem to affect the system.  do you have any ideas.

Answer
hi Tim

If the sound is more like a click it is probably the actuator motor.  It sounds like tic, tic,tic,tic.  Not sure if this is the sound you here.  If the sound is more like a thump thump it is still the actuator but what you are hearing is the door tapping at its extremes.  let me know and I'll help.