Transfer switch

The fireplace probably has a cool down cycle when you turn it off, where the fan continues to run for a minute or so. The electric fireplace unit in my house works that way. It shuts off completely after the cycle is finished.

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.

Doubt the plug change had any effect. I have three and they are all silent. The hum is generally just an annoyance as it should not affect the operation.

I bought a new RV in 2015 and the transfer switch has hummed since day one. I called the manufacturer and they said this was normal. Since I do not have a generator I wired around mine .

OK, The line voltage is 120 volts. The transfer switch is only 2 years old 2016. A friend opened the new head and said it was right.
But the transfer switch is still humming. One other thing that started after this is the fireplace will not turn all the way off the heat stops and the lights dim when you turn it off but won't go completely off. Thanks for your help.

When a Transfer switch hums, it may be normal. It depends on the age of the Transfer switch and also the LINE VOLTAGE. You need to check your line voltage and report back on this forum. I doubt they did something wrong on your replacement head. It is just 4 wires. Doug

No it should not make any noise. I think I would have the new plug checked to see that it was installed correctly.