I have 128k on mine and I've noticed this same slight 'knocking' sounds that sounds similar to a diesel engine. It's always bothered me...but I'm glad to know others get it to and still seem to run fine. I only notice it at idle and under 1k RPM, or when it's really cold out. I heard a C320 recently that was making the exact same noise (except much louder), so its like others have said around the forum, it appears to just be a noisy engine.