The part in the picture is just a cap over the outlet tube from the air (smog) injection pump. It is just a compression fit. See if it is loose by twisting it a little. This could be the source of your rattle. Just pull it off and pinch down the sides a little for it to fit more firmly. The air pump only comes on when the engine is cold and then turns off. When it is on, it does make some sound. If it comes on at other times or makes noise otherwise, it might be time to replace the whole unit.

__________________
03 CLK430 Cabrio 211K Pweter Silver/ Oyster Leather
90 300SEL 214k Pearl Black/ Alto Grey/Black Leather

Prior: 85 190E, 88 300TE, 89 300TE, 90 300E 2.6, 90 300TE, 92 300E 2.6, 91 and 93 300SL, 87 Ferrari Mondial Cabrio