What causes the pinging sound in your grass trimmer and brushcutter and how do you fix it?
The pinging sound that occurs only on idle is the clutch (connected to the engine crank shaft) starting to engage against the clutch drum (connected to the implement drive shaft or cable).
The clutch engages at a certain RPM and works by centrifugal force.
This can be eliminated by adjusting the idle screw counter clockwise to reduce the engine idle speed.
This is normal and it is not necessary to adjust anything if the cutting equipment does not rotate on idle.