Possibly the fuel pumps - that is where they are located on my 87 300E. You might search the archives for fuel pump info as I think previous posters have referred to a buzzing sound from a marginal fuel pump(s).
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