Not disputing at all that it could be the fuel pump(s), mine make a slight CONSTANT hum that seems to be getting a bit louder with time. The OCCASIONAL harmonica note is something new on mine, and just thought I would throw out some alternate suggestions!

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John

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