I could say it doesn't know the words but I won't. Tow probables........There is a low voltage transformer used to power the thermostat and control board. Also the fan motor if out of alignment or about to expire will hum.
My advice: Direct Energy will sell you all of the protection you could possibly want. They come in they clean your furnace they "analyse" it and you pay two hundred bucks. Then; nooooo problems for you.