On many large glow engines, you'll get a knocking sound in the middle throttle ranges. We've seen this on many different engines of different manufacture. We've never seen any evidence of damage to the engine. We can only conclude this is an exhaust note, likely caused by unburned fuel/air charge still burning when it exits the exhaust port.

True detonation will usually happen only at full throttle with a too-lean mixture and bad cooling. This adds a "frying egg" sound to the exhaust note.

For more vertical, you will need to go to a propeller with lower pitch. Increase the diameter to keep the RPM within bounds. As long as you're in the high 9,000's at full throttle on the ground, you'll be OK.