I have a few Cadet wall units in my house. (These are about 12x8 in units mounted in the walls.)
One of the units has a built-in thermostat and a knob to adjust the temperature.
I don't know how old the unit is.
The strange thing is that often after running for a while the unit starts making a hissing noise. Not a buzzing but something higher pitched, like as if steam were being released from a valve. (But I don't believe there is any liquid involved here.)
My best guess is that the hissing happens when the temperature reaches and thermostat value, so the heater turns itself off, but then some other mechanism with different calibration decides it still needs to be on. And it goes back and forth very quickly. But event then I would expect to hear a bunch of fast clicking sounds, not a hissing sound. (One other detail of the hissing is that when it's in that state it can mess with my computer mouse -- like the mouse doesn't work, and then I adjust the thermostat to stop the hissing, and then the mouse works again. So there is some electrical strangeness happening.)
So one question is where the hissing comes from.
But the more important question for me is that I was recently diagnosed with a life-threatening form of sarcoidosis. It's too late to undo that now, but for sanity's sake I'm trying to understand if this misbehaving Cadet heater was partially responsible for my illness. They don't know what causes sarcoidosis, but among the few possible triggers they list are various types of inorganic dust, especially metallic dust.
So the real question is whether this hissing heater could have been spewing out aerosolized bits of metal that I was then breathing in. A friend says the heat would not be enough to break off tiny pieces of metal, but I don't know.