How are your tires? I had a whining noise coming from the trans/engine area at 45-50 mph on my C230 (1998) which the dealer said was 'normal' - it almost completely disapeared the very day I installed new tires. There was plenty of tread on the old Goodyears - I switched them to go to 16" rims. Could tires make a noise like that only when warmed up?
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