Ah. Well I would suggest you rejet anyway, as the engine will be running lean and hotter than it should.
This may be the cause, as if the engine is running hot enough it will pre-ignite the mixture. To be honest though, I would expect you to notice the heat. Have you had the plugs out to check the colour since you ran it?
The most likely cause is an air leak in th exhaust system. Check that the exhaust headers are properly seated and sealed at the cylinder heads. I imagine the noise happens just as you throttle off, in which case the system is sucking in air from somewhere.
Have a look and let us know how you get on...
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