Is it normal to hear air coming from i guess the ventilation system
1994 c200 ? non a/c (England) . The noise stops when i put my foot on the gas but returns as soon as i take it off, Thanks Richard.

Sounds like a classic vacuum leak to me.
When you accelerate, vacuum ( and noise ) drops, only to return at off-throttle.
The leak could possibly be in the dash area.

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2007 C 230 Sport.