faint ticking sound in dash (W210 E420)
 

I have this annoying ticking that come on inside the dash. It is not the oil lifters sound and it is not coming from the blower motor. Any clues? I do not seem to be able to narrow it down.

Does it stop if you turn off the Climate Control ?

I have it too, and mine stops when climate control is turned off.

What does that mean?

It stops after I turn off the ac (turn the blower to OFF, have not tried putting the ac in EC), but there is about a 10 to 20 seconds delay. Seems to have nothing to do with the blower motor, even though that was my first guess.

The first suspect in that case would be the little aspirator fan motor that samples the air for the CC system.
Try the search function to find out were it is located on each model.

that suspect would not be it on the W210, the aspirator fan is on the ceiling by the rear view mirror. What would be another thought?

Blower fan blades could have leaves/pine cone needles in the housing is another possible ..

Some of those aspirator systems air exchange is up where you mention, but others have a tube that the air comes out..the actual motor is down below...
The 210 Sampler is in the overhead light console, as you suggest.
And most sampler motors screech rather than tick.
The blower coast to a stop after CC is turned off, so that is a likely possible. Try just the blower with CC in EC.

May be one of the vent doors behind the center console....?

Hmmm. Why would the vent doors click, do they get loose?