macavity, if you had a '86-'9? and the noise was on the right(passenger side) I would say it was the vacuum pump for the locking system. But in 12 years I've never heard mine.My fuel pumps(yes pumps, my 300E has 2)are on the right side outside underneath in an enclosure. Yes while it is in warrantee have it checked out. Happy Trails Beep Beep from Houston.
1987 300E 169,000mi.in service 1988
1988 Merkur Scorpio 48,000 mi. in service 1988