Transfer switch buzzing

Transfer switches buzz because the two metal armatures are not 'sealing up' tight together. Dirt will find it's way in and AC relays do not like anything between the two surfaces. If an AC relay does not 'seal up', it's coil will draw extra current and a buzzing relay will be much more likely to burn up. If you feel capable, you can take the relay apart and clean the two laminated surfaces. Putting a piece of fine sandpaper on a flat surface and rubbing each armature across it to clean and flatten the surface will help. When the surfaces get really beat up, you'll need a new relay. Just something else that will last much longer if you maintain it by keeping it clean. If it isn't buzzing, it's not getting damaged.