My 1987 300E does that as well sometimes - started doing it a couple of months ago. Speculation by my tech was that the fuel accumulator may be going bad, but to try installing a new gas cap first to rule out a cheap source of possible noise. I have installed a new gas cap but conditions haven't been right for the sound to appear. Usually it shows up if the car has been run for a while on a warm day, parked for 10-20 minutes, then re-started. As it gets into the 70s next week, I may have a chance to see if the cap fixed the problem. It would sure beat paying for a new accumulator.

http://www.peachparts.com/shopforum/showthread.php?threadid=56563&highlight=bubble

__________________
John

2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)

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