Have a persistent knocking sound from the engine on a 1991 300E with 137K miles. The frequency of the sound varies with engine rpm and is present at all engine speeds. The sound is coming from the right side of the engine, in-line with the front cylinder and about halfway up the block (close to were the vacuum pump bracket attaches to the block. I removed the serpetine belt and the noise is still there. I removed the valve cover and everything looks normal. I ran the engine with the valve cover removed and the timing chain was not loose or slapping. The sound is in the vicinty of the timing chain tensioner, but towards the back of the engine about 3 inches. The sound does not change when I disconnect the spark plug lead from the front two cylinders. Help, what should I do next...