Here is the deal.

when stopped with the engine running (regardless if I am in gear or not), I can hear a tikcing noise...like a clock......which seems to be coming from somewhere behind the center A/C vents or behind the radio...(ya know that area....in the middle)...

When I put my hand on that area...I can feel it pulsating as it ticks...

It is a tick that comes on every 5 seconds ONLY when my A/C or heater is on (and both work A okay....actually the A/C is more like a freezer, and the heater...will get you sweating)....

Very weird thing!!..

My car is a 1996 C220, 74K miles.

I would appreciate any ideas.....no matter how weird they are...they will not be weirder than the problem itself.