Before I ask this question, please forgive me of my ignorance of 4 strokes, as I raced 2 strokes my entire life until I made the switch to a 2004 YZ 450 back in November and am pretty much clueless. Here is what is concerning me. I fired-up my bike tonight before I changed the oil and filter like I do after every ride, and I don't know why I did it, but I stuck my head down next to the outside of the cylinder to listen. Every time I revved the bike a little I noticed somewhat of a clattering/knocking sound that got progressively louder the higher I got into the rpm's. Is this normal for 4 strokes? Do I need to post a vid of it? Any advice would be greatly appreciated.