Everyone says these engines rattle and I was thinking of why that might be. I'm not sure of the construction of the newer KDX engine power valves but in my bike there is a "wave" washer (that can be compressed) backing up the center valve and I was thinking the expanding/burning air/fuel mixture (when the piston gets below the top of the exhaust port) could be pushing back the valve and either when it retreats or when it rebounds (or both) it could be causing the annoying sound as metal hits metal.
What do you think?

Isn't it more like a knocking sound?

yeah it is
do you think the valve banging the cylinder would make that type of sound?
I do
I want to stack two wave washers together and see if that changes anything but I need to order one first.

It's interesting that after all of this time there is not a 'fix' for the noise, it seems that everyone knows about it and just accepts it. It's not a good noise, but obviously does not affect the reliability too much. (the KDX has an awesome reputation)

I know what the sound is on my bike and why it happens.After having the clutch apart several times,I noticed that the clutch basket(when assembeled on the input shaft)has a ton of play in it.That's up & down and side to side.At first I thought it was the carrier bearing.But closer exam and some thought tells me that it's end play.I have a manual and still do not understand what that shaft sits in.Some type of cup bearing in the left side cover?I've never split the cases on a KDX.I know this is the noise because I can mimic the sound by moving the clutch basket by hand.That's why the sound is really noticable at idle and gone above that.If the basket was perfectly balanced it might not make that noise.If I ever split my cases,Ill take a closer look.In the mean time it's the same old reply(s) to the same old ?'s.Buy the way,it was like that new(01)and is the same now at about 2000 miles.

I solved the engine rattle in my '02 220. Go back to the 1st page on this forum and choose the top thread "Important Poll ...." there is a discussion about this on the link shown at the bottom of the second post. I have heard two distinct rattles coming from my bike - one in the cylinder and one in the clutch. I've heard the same noises in KX's and in KTM's. If you think a KDX is loud, the KTM's are twice as loud straight off the showroom floor (clutch noise). The top end rattle in my bike is caused by carbon build-up on the main "flapper" valve. I can hear it from low to mid rpm. When I cleaned this part, the noise stopped. When I corrected the jetting on the bike and swithched to Mobil 1 synthetic pre-mix (40:1), the noise didn't return until after 400 miles but it wasn't as bad. When the bike was brand new (jetted rich) I ran Yamalube with the stock jetting. Yamalube is a great oil but with the stock jetting my main valve was caked with carbon, and the noise was terrible. I can only speak on my experiences with this bike. The noise you're hearing could be for another reason. Clean the valves really good, and take it from there.

The clutch noise is easy to distinguish. At idle, pull in the clutch lever. If the noise changes tone, you'll know it's a typical KDX. Try putting in a full quart of 10W-30.

Porter, the bearing at the left end of that clutch shaft is a needle bearing and I found that they go bad easily if you don't keep enough oil (and of good quality) in the gearbox. Take the clutch off and move the shaft up/down to see if the play is in the near or far left bearing.

I'm pretty sure my engine noise is from the engine and not the clutch.

Actually, there's 3 sources of all that noise. The power valve when assembled
simply has lots of "slop" in it [probably so that it will still function when the owner neglects to clean it]. you can take a freshly assembled cylinder, shake it and you'll hear the valve rattling around. The power valve in my KX also rettles like crazy. The second source of noise is the bottom of the cylinder. There's just not much bearing surface down there and the piston quickly wears in that area and you get piston slap. The third , as mentioned is the clutch basket. It's a very poor casting and no there's signs that they tried to balance it. As soon as my clutch lever is released you can hear - and feel - it wobbling about in there. There's probably both axial and radial play. I replaced the bearing just behind the clutch hub but with no decrease in noise.