My 2001 CLK55 (120K miles) makes a loud clicking sound while driving under acceleration at low speeds, more so when the engine is cold. The sound is in perfect synchronization with the engine. It is intermittent right now, but seems to be getting a little worse every day. I cannot duplicate the sound while in neutral, only like I mentioned, which is generally at low speeds (2nd or 3rd gear, 40 mph or lower) when I open up the throttle to accelerate. I have noticed the rattle (or loud clicking) at higher speeds (above 50 mph), but not as much as during lower speed. I took the car to my mechanic and he listened with a stethoscope all around the engine and transmission for anything unusual, but found nothing. He put the car on a lift and engaged the gears, with the tires moving as he did this, but didn't put any real load on it since it was not on the ground. The car still runs fine, I haven't noticed any dropoff in performance or gas mileage. It shifts just fine too. As is usually the case, when I tried to duplicate the problem with the mechanic, it wasn't making the noise as loud as it sometimes does, and he didn't really hear it. He did notice that one of the spark plug wires had been damaged and was arcing electricity between the damaged wire and the heat shield next to it. He originally thought that might have caused the rattle, but he fixed the spark plug wire and the issue still exists. I'm far from an expert, but I don't see any obvious signs pointing me in the catalytic converter direction or the transmission/torque converter direction either. Has anyone had any experiences like mine?