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Thump a thumpa sound in the dash behind the radio

I have a '97 Lincoln Cartier Towncar which has 102000 miles and has been a dream, the best of 6 I have owned.

Last week when the weather turned hot, I started the car which had been parked outside and heard a thumping sound behind the radio, which has a tape player and a 10 cd player in the trunk.

The sound is like Thumpa, thumpa, thumpa and stopped after I turned on the A/C.

It seems to get shorter each time it happens

Any ideas ???

There is a motor that controls the blend door in the HVAC system. The door controls the air flow across the heater core or bypassing it. The door is stuck and the stepper motor keeps trying to move it. You probably need to have the door replaced. Eventually the thumping will stop after the motor burns out.

Yea, most likely the blend door.  I have also heard heater fans make noises like that.  Try changing the fan speed (some run even when turned off) and see if it changes.  In both cases it ends up disassembling it anyway.

Thanks for your ideas… appreciate it.
Herb Vincent