Have you pulled the valve covers and looked at the chain and the rails. I bought a 420SEL (170K), this summer and that's the first thing I did. My chain had slop and part of the left upper rail was laying on the chain. I stopped driving it immediately. With the help of these guys, I replaced the chain, the tensioner and all the rail guides. If you drive like I intend to (5k/yr), you won't have to worry about this ever again. Cost me my time and about $150