2016 yz250x

Hello, so I am having wat to me sounds like a combustion issue she idles and revs in N nice she goes good once I hit 3rd u can hear it in tge cylinder head It gets faster as u speed up and it continues doing it until u reach a complete stop then basically rinse and repeat it also continues doing it even at as it coasts but doesn't seem to get faster with the rpms any possible insight would be muchly appreciated thx

Ok so when I fire up my bike she idles good runs good till about 3rd gear then it starts sounding almost like it's not combusting properly almost sounds like an extra puff of air kinda thing each rotation of the tires it's kinda hard to explain