I would take the cover down under the passenger side (two slotted quarter turn screws), where the recirculation air filter is, and take a look make sure the recirculation door is closing all the way in normal mode. I had changed that filter once and didn't get it seated all the way, and the filter cage was keeping the door from closing, causing symptoms similar to yours. Do you get a whistling sound at higher blower speed levels?

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1983 380SL
1995 C280
1995 S500 coupe
1990 Mazda Miata
2004 Suzuki Hayabusa