I have a simular problem on mine with the strange sound comming of the engine. Only happing when you hit the pedal completly. I have it in different gears, but it always starts after a gear change when i hit the pedal.

When i keep my foot down the sound stays, rpm stay exact the same. When take foot up and down sometimes it goes again sometimes i need to hit it slower and it goes.

Does this match your problem? or your is different