I have a '85 190 E and when I have the heater turned on there is a clicking sound coming from what sounds like the passenger side, in the dash behind the vent. Now, when the temp guage is turned up all the way there is no noise, but if I lower the temp...a clicking it will go. It doesn't do it all the time and the clicking stays at the same noise level. Any ideas of what I should be looking for or where to even start? Thanks for your help!

 

locate it first as near as you could.

it can be the vacuum element.

 

Normal operation for the heater valve. closed during cooling, wide open at full heat, pulsing when regulating warmer temperatures, hence the pattern you are hearing.