I've noticed for a few years that there's a sort of knuckly knocking sound from my car when idling. Nothing particularly loud, but I put it down to ageing and cold weather. Since repairing the LPG injection system after almost a years' down time, I've noticed that the sound disappears completely when on LPG, and is immediately there again when I switch it back to petrol. Does that mean injectors make a noise? Or is there some subtle difference between how the two fuels burn?