If you face the engine there is the center plastic cover with merc in silver in the middle. To the left of that is a pipe running from the front of the car into a black plastic box on the exhaust side of things (above pre-cat and turbo). On the left hand side to the back of the box is a knob.
It may be the air filter box and the knob you are talking about could be the filter indicator that turns red when the filter needs changing and then you turn anti clockwise to reset when the filter is changed.