Help please - strange noise EC 300-2019

[Doc Brown]

Today I heard a strange noise, that only appears when the engine is hot. When cruising in any gear as soon as I close the throttle I hear a knock. It is one single knock.
The bike is stock, except that I installed a GP head with 1,8mm squish (instead of 2mm of the stock head) GG says use 98 octane fuel but I only could get 95. And it never did ping/knock before with 95 octane gas.

The knock is only audible when I ride with low rpm's in any*gear and it is only audible the moment I close the throttle. It is pretty loud. I checked a lot of things but I fear it comes from inside the motor.

I called a GG dealer and friend in Germany and he says the GP head has too much compression for the EC 300. But to be honest that's hard to believe as piston and cylinder of both engines are identical. I measured 2mm of squish with the standard head and 1.8 with the GP head. But he says the squish aint the problem, its the combustion chamber which raises the compression too much.

My OEM head has been on the lathe and 0,5mm been taken off, creating higher compression and 1,55 squish. So I took off 0.25mm of the squish band and also took a little bit off the combustion chamber. I will try this head and check if the capacity of the chambers off both heads differ.

If it is not a typical ping, what else could it be? And why would the engine knock/ping only once in exactly the moment I close the throttle. Bike runs fine, has good fuel consumption and rips like an animal...

(will crosspost to TT)