What is this noise?
My 400ex does this same noise, and only has been ridden for about 55 hours. It's very annoying.

*Not my video*

 

1) What mods do you have?
2) When was the last time you adjusted the valves?

 

Twin Air Filter, otherwise performance stock

 

When was the last time you adjusted the valves? Does it make the noise with the stock filter on?

 

I've never adjusted them. I think the sounds comes from the top of the cylinder, and not the carb.

 

Put the stock filter back on and see if it makes the noise. Mine did made a similar sound when put a uni filter on and did not rejet the carb. She was running real lean with just a air filter change. We you run lean there is a tremendous amount of heat created and it can burn up your top end.
It would be a good idea to adjust the valves.