It sounds like A fuel pump to me, I believe there is more than one.
The fuel pump cycles for about 1-2seconds before starting the car when the key is in the last position before cranking. I've had them go in 2 of my cars, but I never had the luxury of any advanced notice.
2003 S500 Black/Charcoal
1990 560SEL 61k Arctic White/Grey
1988 420SEL Black/Palomino Sold @ 85k
1987 420SEL Midnight Blue/Grey
1986 420SEL Diamond Blue/Grey
1983 380SEL Champagne/Palomino