13 250xc Funny Noise

I would remove the exhaust pipe and right side power valve cover for inspection. Very minor disassembly. While going thru my 14 250SX rebuild, I removed the same cover and found the power valve spring with zero tension and not connected. Assuming my power valve wasn't even working correctly, funny cause I didn't notice it.

Sounds like right side crank bearing worn out, to me.
I had this on my 2011 250sx (same generation motor), where I had recently rebuilt the top end, checked the rod for play, felt tight, 5 hrs later, right side bearing failed after noticing similar noises at the practice track and then "sending it" the next weekend at a race.

I learned that you can check rod play on these two strokes by checking the flywheel for up and down and in and out play.

Lifting the cylinder is another way to check, but I would suggest you check the flywheel too. That's the part I failed to check.

As the bearing starts to go, the jetting will go off as the crank starts to walk and opens the right side crank seal.

It will intermittently suck in air and trans oil from the right side crankcase, which messes up the combustion/jetting.

You might also feel vibration from the crank starting to walk inside the bearing races.

This shows the topend rebuild: https://thumpertalk.com/forums/topic/1163810-ktm-250300-topend-rebuild-…

I will see if I can find a link to my engine rebuild, after replacing the piston.

There is a guy selling a core (crank needs rebuild) from a 2016 250/300 XCW on thumpertalk. Let me know if you need a link.