On cold starts my 94 E320 engine (100,000 miles) makes a just barely audible from the driver's seat 'clicking' sound that seems to correspond to engine speed. The sound stops when the engine reaches operating temperature. The sound was there when I bought the car at 81,000 miles, and has not become any worse during my ownership. I use Mobile 1 and change it every 3,000 miles. Any ideas?

Mark