Strange "clonk" inside dashboard
 

Hi all,

this is my first post in the forum. This is a good place.

I own a 93 400E all service records with 200.000Km. From a few days ago, I am hearing a strange "clonk" inside the dashboard (like a little door closing and opening), is sounds related to the air conditionong system, it seems to change with inside temp change, sounds loudly at first and then low when hot, it has a rithm like 3 seconds between "clonks". Any idea on cause of this. Thanks a lot from Spain.

Mine did this a few months ago. Loud 'thwunk' sound at 2-3 second intervals from the vents, fading out after a few minutes use. I made an appointment to see my tech, and the sounds stopped before I could get in. It only did it for 5 or 6 days total. He said there were a number of doors and vacuum components in the AC system which could be responsible, but if it wasn't doing the noise right then, he wouldn't go into the dash, and suggested waiting until something got noisey again or stopped working. Of course it hasn't had a problem since!

Partially Solved!!
 

After looking for any item related to A/C , I have discovered I was running in low level of coolant, so I have refilled and the strange sond has gone to a low, low clonk really hard to discover.