HEy i have the same issue you have right now, been trying to figure out a solution for the past 2 weeks and have read many threads on here that did not give a clear solution. Mine may be coming from a different part, however my noise is coming from the actuator the opens and closes the recirculation door from the outside, the noise only comes on when the door is closed and pressure in the vaccum lines is keeping the door closed. I have gone to the dealership in my area and the part in $50 and even cheaper if you buy it from the mercedes dealer and just explain what part you need and a picture that i had the dealer print off for me. Many people said they have fixed the issue just by tweaking the vacum lines going to the actuator from the recirculation air switchover valve i have a pic that another member posted :

The arrow points to the part that is the culprit of the noise, this is all behind the passenger compartment just FYI. I cannot figure out how to remove this actuator at all, if anybody has any information please let us know thanks. I also have the part numbers from the dealer for the part as well that i will post later - Tim