jacksbike

I am not totally familiar with hand driven cycles, but I will give this a shot. Since you said that the "ponging" sound occurs even when you are not pedalling, then it sounds like a hub problem. Check the hub axle and see if it needs adjusting. Perhaps the axle cones are adjusted too tight or there is a bearing problem -either needs greasing or the axle cones are worn. If there is a plastic or metal spoke protector behind the freewheel (is there a freewheel on the front wheel ?) maybe it is rubbing against the spokes. Sometimes the hub threads that a freewheel threads onto is not perfectly straight and the freewheel wobbles around the hub -possibly rubbing against the spoke protector. Another idea could be spokes rubbing against each other where they cross. Is the wheel tight in the frame or is it shifting slightly as the bike moves ? Check for tightness. Possbile rubbing of the wheel against any brake shoes ? Here are my suggestions, hope it helps you.