Weird whistling noise coming from my ATV engine?

So i have a honda 250ex, and today whenever i would give it gas, i would here a strange whistling noise coming from the cylinder block in the engine. When in neutral the sound was not heard. I never heard this sound before and it made me a little nervous. Just wondering if there's a problem with it or if i need to replace something.

You could have an exhaust leak or the manifold is not tighten properly