1992 Vanden Plas.

Whilst the AC is on cold, there is a loud, rapid, flapping noise that
seems to have its origin at the center AC vent.

If I turn the temperature control to maximum heat, the noise goes
away. Whilst the motor is under acceleration, producing torque, (such
as pulling from a dead stop, etc.) the noise goes away. However, just
revving up the motor while in neutral does not make it go away.

I disconnected the vacuum line at the point where it enters the
console beneath the battery and the noise disappears.

All this indicates a defective vacuum actuator somewhere under the
dash.

If anyone has experienced this problem or can provide any tips on
fixing it, I would greatly appreciate your sharing.

Cheers!

John