Nope, mine is not related to the gas pedal. Actually, on the way home from work today I realized it isn't really occurring quite as frequently as I originally stated; more like once every 3 or 4 minutes if I recall correctly (it's hard to concentrate on these things and keep track while driving). It's a hissing noise that lasts about 2 or 3 seconds; starts out strong and then fades out. It is truly cyclical so it appears to be intentional. I'd like to know what it is...

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280