Tried this on the technical forum, but perhaps it is better suited here...

Hi Folks:

W209 (clk 320). I bought the car a couple months ago and since have noticed this annoying...almost muted tapping noise coming from the engine. Faint at idle and increases upon accleration - most noticable at 2500-3000 RPM. It's driving me crazy. When the windows are open, I don't hear it as much. When I close the windows, I can hear it more. Here are the parameters:

Happens hot or cold
Air conditioning on or off
Oil changed to MObile 1 0W40 today, no change
Dealer replaced engine mounts
Dealer replaced Idler Pulley
Dealer says they took off the serpentine belt and noise still exists, appears to be from the valve-train and is within normal range.
Use 92 octone fuel from Chevron
It appears to be louder from inside the cabin

Any thoughts??