I have a 1998 ML320. After a recent service, I've noticed several problems with the car. These included the following:
1. Dead battery - replaced battery
2. Dead car - determined not to be the battery, however car would not start. Replaced ECM and ? second computer module. ($1800)
3. Ticking in dash behind vents above and to the right of the radio. The ticking was intermittant before and now is constant. It is of constant frequency and nothing seems to change the noise. It occurs in the second key position but not in the first as well as when the car is running. Ticking still present after ECM replacement.

What is in the dash behind/to the right of the vents that could make an audible click heard only in the car (not under the hood)?

Thanks

try moving the temp gauge to different positions. I think have the same noise...... it is an incomplete closing of a vent.... it just keeps trying. repair is not at a reasonable cost. the book lists 16 hours labor!!!!!!!!! Mine was intermittent for a while but now is almost all the time if I have it near max cool temp. Stops a few clicks off max cool. I hope for your sake this is not the same noise as mine!

The noise comes from a "stuck" blender motor or similar name. It will vary with the temp control as the motor moves the flap to allow cool/warm air.

I had the same problem on my wife's 99ML. Part costs around $40 but labor is around 5 hours (not 16)

I had it fixed at the dealer and the total bill was just less than $500. I always worry about dash removal jobs but it went back together fine and no rattles as a result.

My opinion is that its NOT an easy DIY fix. I managed to see the problem by removing the glove box insert and shining a flashlight inside but I couldn't see an easy way to get to the problem. I think that the Techs remove the top half of the dash but I am not sure.

Sweet relief once fixed though!!