My 420 was doing that, but only when started cold. Once the engine warmed up, as designed, the air pump no longer ran, so no more noise. I unplugged it, since my state does not test for emissions, and had no more problems.

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-- Paul W. (The Benzadmiral)
('03 Buick Park Avenue, charcoal/cream)
Formerly:
'97 C230, smoke silver/parchment; '86 420SEL, anthracite/light grey; '84 280CE (W123), dark blue/palomino