When I'm driving my W126, there is a whistling noise that seems like it is coming from the dash...perhaps the defroster vents. I can shut off the HVAC and it's still there. It also seems to flucuate or change with RPM. Not in a linear fashion, but RPM does have an effect on it. It's not terribly loud, but noticible. Any idea what this might be?

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'86 420SE Euro
904 Midnight Blue, Gray Velour
Dad bought it new, now I own it.

"A Mercedes-Benz is like a fine wine, it only gets better with age."