I just got a carrier furnace installed yesterday. I notice that when the furnace is off (thermostat not prompting for either heat or fan), there is still a moderate buzzing/humming noise. If I switch off the electricity, of course the noise goes out.
I called the installer, and he said that this is just the electrical noise which is present in all systems.... I do not need to worry about it. While the noise is not loud (especially after I close the furnace room door), I still am a little unconvinced that it is normal. Does your furnace make this slight noise in the off cycle as well?