Hello folks I've been reading for the last year and have learned alot. You are all my source of info... I have 2003 DRZ400S. I have put on about 4500 km (2300 miles) last year and things seem to be great. However, since about 1000 miles, I somtimes hear a slapping sound from down below ...don't get the wrong idea. It sounds like it might have been the chain, so I tightened it up. Still have the sound. Thought it might be motor knock, so I used Premium gas. Still have the knock. Any ideas ??? Thanks in advance.