Ok its making the sound but I was at a stop sign and it stopped.

How often does the air pump cycle?

Only at cold start up for about the first 30 seconds or so until the cat warms up. It's only purpose is to lower smog emissions at start up.

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".