It's possible you could be hearing noise from a small vacuum leak - you might try temporarily blocking off the vacuum line to the climate control system to see if the noise goes away.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C