I have an '06 250x that makes the same noise. My guess is that it is the cam chain. Either the cam chain needs to be replaced, or the tensioner has failed. Apparently the tensioners do fail on these bikes, and it is the easiest to check as you don't need to open the case to check it. Otherwise you can remove the side cover on the left side and you should be able to see the bottom of the cam chain. If it's loose then that's the problem. As far as I know, of you remove the valve cover and the side case, you can remove and replace the chain. If you also need to replace the guides, then I think the head needs to come off.