2000 W210 E320 whistle from fresh-air setting

[nazrat]

Our 2000 E320 has developed a whistle noise from the vents that varies with the fan blower speed. By selecting recirculation the noise is completely gone.

I'm guessing that the blend door isn't resting where it should when it's set to allow fresh air in.

I check the filters. Removing them made the noise louder - I believe because of the increased air flow.

Any ideas?

-Tad