I bought my car as a daily driver about a month ago for my 110 mile a day commute. This is my first MB with ACC. The problem is that when I run the car with the EC button pressed, a noticable flapping/knocking noise can be heard from the passenger footwell. I thought it was the blower fan so I replaced it with a working unit from a junkyard. However the noise still exists. However, if I press the full ac button when I hear the noise, the noise goes away. This noise is very intermittent. Could this be a problem with the fresh air vents? My previous MBs (240D and 190E euro) had manual climate controls and I never had this problem. Any ideas?