The "hard to explain" sound part is easy to fix...take your cellphone and record the sound and either post it as a video or take it to your favorite MB tech and have him listen.
Either way, someone with the right amount of experience should be able to tell your it's "Toast" or "Wundervoll!"
Inexperience, on my part, says that not enough oil is getting to the main bearings on the crankshaft...but then, I'm no expert. I slept at a "Sleep Inn" last night.
Really!
