Originally Posted by RML

When I get in my 85 300D and turn the key to the glow plug position, while I am waiting for the yellow glow plug light to go out, I hear a heartbeat sound coming from behind the dash. It happens if I have left the climate control on in the vent position (second button from the right). I think it also happens if I have left the A/C button on, but I usually turn off the A/C when I shut down the car. If I turn off the climate control by pressing the right-most button the heartbeat sound stops. I do not remember ever hearing this before. Has anyone else experienced this or have any clues as to what is causing it?