Most propane appliance regulators use a rubber diaphragm to control the amount of fuel allowed to the control valves. If the propane tank has been overfilled, or it is a hot day outside, it is possible for this rubber diaphragm to vibrate. As it vibrates, it creates a humming noise. This noise, although annoying, is perfectly safe, and will go away in time. If the humming still persists, you may want to order a new regulator.