My 1986 420SEL was real quiet running this morning, Then I started it up this evening and now there is a "marbles in a tin can" noise coming from the front of the engine and it's louder with the oil filler cap off. Is this the timing chain slapping? If so, how big a job is it to fix? Is there any step by step instructions on how to replace the tensioner, guide rails, etc.? Or should I just bring it to a shop and get hit with a big bucks repair job? Could it just start this noise all of a sudden? Wouldn't I hear this noise lightly at first then getting louder over time????