I recently noticed that my 1979 240D has developed an erratic noise at cold idle when first started up. Every few seconds there will be a ringing noise in among the idle clatter, like someone striking a large resonant bell with a small hammer, kind of like one of those Japanese bells. It seems to go away once the engine has warmed up; if fact, I had taken my car in for a coolant pressure test at MBI here in Portland, and they alerted me to the condition, but with the car warmed up at the time I couldn't even detect that something was wrong. The next time I started it up I heard it clearly, however, and to be cautious I have stopped driving it. MBI was of the opinion that one of the chain tensioner guides has broken, though when I ran this by the shop that rebuilt the engine in 2011 (Bearing Service) Brian seemed doubtful.
I figure that if I pull the lower oil pan I'll either see plastic pieces or I won't, and that may settle the diagnosis if I do.
Other random facts:
The 2011 (2012?) rebuild was very complete: new pistons, rings, oil pump, full head work, valves ground, crank polished and balanced, connecting rods offset bored to size - basically I told them not to cut any corners, not wanting to have to go back in for a long time. At the time of the rebuild I put in rebuilt injectors and had the injection pump professionally rebuilt. Well, it's been a fairly long time, but not very many miles (30k!), and here we are. I've given it a yearly tune-up every fall with valve adjustment, change of oil (synthetic), change of fuel filters, change of air filter. Were it not for this worrying sound I would say the engine is in tip-top condition, with plenty of power (well, plenty for a 240D
). Thoughts?