Noisy Transfer Switch

The transfer switch is really a relay and all mechanical relays are subject to chatter. It can sound like humming but humming is a little different. Swipe the core of the coil where the armature hits it with a piece of coarse paper, a brown paper bag strip will work. It may help but probably not. The problem may be a loose coil, (humming) a worn armature or just a lousy relay engineering wise (vibrating). We had tens of thousands of relays in traffic signal controllers and we found the only way to make sure they were quiet was to buy really expensive replacements or as we finally did, we changed to soft start solid state relays.