When I step on the gas I hear a humming noise

When you accelerate your engine is normally twisting a very small amount within the confines of the engine mounts. It is possible that either the engine or the exhaust pipe is touching something at that time. To begin, look for a heat shield that is very close to the exhaust. Sometimes the catalytic converter (part of the exhaust) can make this noise when it is defective, an exhaust shop can test it for you.