Power Loss

[Clockwork]

Hello Fellow GZ250 enthusiasts.

I am wondering if somebody can help me diagnose a problem with my Bike. When ever I am driving, after about 15 minutes (presumably the time it takes the Engine to heat up) the bike looses a lot of it's power. This is most noticeable at high speeds. It happened today (First time taking it above 60kph/37mph in a long time), After driving along at 80kph/50mph for 10-15 minutes, the engine makes a noise that is hard to describe. It is sort of like the sound of fine grained sandpaper being used, or like air being sucked through a crack. With this sound also comes the power loss. The bike slowly looses speed. When this happens, the bike can only maintain 70kph/43mph in 5th gear and can't get above 60kph/37mph in 4th. This problem persists until I turn off the bike and let it cool down. Once cool, the bike returns to normal and the power comes back.

My thought is that there might be an air leak between the carb and the head which gets worse with heat. Perhaps it is messing up the vacuum, so not as much fuel is being drawn into the carb. Does this sound possible? Any other thoughts?

Thanks

Clockwork