Could it be your fuel pump? an SL owner describes a whine from the rear of the car in the following thread:
Buzzing - fuel pump?
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E