Jan 07, 2020 - 06:23 AM
The ATS 40250 only has red lights. Those lights should be illuminated red when unit is plugged in. They will never turn green. The hum is from the coil inside the relay which engages the switch. The hum is normal. It is similar to a florescent light fixture hum or a transformer on a telephone pole hum. It is nothing to worry about.