Transfer switch

Transfer switches hum because they are 120 volt AC relays and they will do that if the 2 steel cores inside do not mate up tightly. In an RV this is usually caused by dust/dirt buildup between the 2 surfaces. It's a PIA but every transfer switch I have ever seen can be carefully disassembled and those surfaces cleaned. If it has been buzzing a long time, the surfaces will get somewhat beat up and require a little 'dressing' with a file. Not as easy as it sounds as the two surfaces will have gotten 'work hardened' from banging (and buzzing) together and be difficult to file. Often when they start humming, you can unplug and replug and the two surfaces will align just enough differently to stop buzzing. As it gets worse in time it will cause the coil to overheat and either burn out or damage the plastic parts. AC relays rely on a tight 'sealing up' to minimize current flow through the coil. A little maintenance can help save an expensive pair of relays.