check vacuum actualtors- they might not be closing all the way-
try to let air go throughother vents ie: floor, windshield, then center vents, if for some reason it goes away when a certain vent closes, then it's the vacuum actuator that controls that certain flap- it might be stuck @ semi-close that would create a whistling sound.. hope I made sense

a2

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a2


1978 280 CE silver
1986 560 SL black
1987 300 SDL cream
1994 SL 500 brilliant green
1997 E 300d dessert silver
2005 FORD F250 Superduty Crew cab 4x4

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