AC whistling noise

[sleeping_doc]

I have a 2001 E320. Intermittently the AC would emit a high-pitched whistling blowing noise. I have taken the car to 4 dealerships but none of them could tell me what's wrong or to fix it. Help. One dealership even told me that's the sound generated from the AC trying to cool down the interior of the car, what a BS. I have other MB with the exact AC system but no whistling.