What causes the pinging sound and how can I fix it?

The pinging sound that only occurs at 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 the certain rpm and works by centrifugal force. You can eliminate this by turning the idle screw anticlockwise to reduce the engine idle speed.
This is normal and it is not necessary to adjust anything if the chain does not rotate on idle.