Why are the gauges bouncing up and down

Hello, my first gut feeling is that you have a charging system problem, such as with your alternator. That assembly may also be causing a humming noise if there is an internal problem. It can cause electrical problems that can prevent proper idling and make the gauges act erratically. All that said, it could be something else! Testing is needed and a technician's good ear can help determine the source of a noise.