I have a 2005 250ex making a noise that I 1st thought was a valve ticking. I set the valves, the tick is still there. Upon further inspection, the tick appears to be coming from the bottom end... almost like a rod knocking, but not as deep in tone. Also, I've always changed the oil and there really is no reason for the rod to be knocking. I've looked at some schematics and noticed a chain driving the oil pump, yet there doesn't seem to be a tensioner. Is it possible the chain is loose and smacking around? Are there any known issues with these engines?