We are in a house in Chandler, AZ, that is about 4 years old. It has forced air gas heating. I hear that it's been done cheaply: both the heater and air conditioner are located in the attic (the a/c compressors are outside, the blower etc. are in the attic). People say this is a bad design but cheaper for builders, so we're stuck with it.
We bought the house in during the summer, so we tested the a/c and it works flawlessly. I also tested briefly the heater enough to notice that it worked, but I was so excited looking around our potential new dream home I didn't notice the high pitched squeal it makes.
So around November, when you have to start using the heater in this part of Arizona, I switch the thermostat over to heat and am appalled when I hear this high pitch whining noise that appeared to coincide with the heater. I figured, no problem, it's probably an unlucky small vent in a duct that's making this noise, and I'll just find it and go over it with duct tape. But then I notice that it doesn't coincide with when the blower runs.
Here's its behavior: when the heater lights up to warm the air BEFORE the blower starts, the whine begins. It continues while the blower is blowing, but then about 2 minutes before the blower stops it stops. I assume this is the same time the fire goes out and it stops actively heating while the blower continues to blow over the still hot metal parts.
Does anyone have any idea what is doing this? I could swear it seems to be coming not from the heater itself but from some ductwork about 20 feet away in the attic (inaccessible to me), and this makes no sense to me because as I said, it doesn't start and stop with the blower, so I don't think it's some kind of air movement noise. (I could be mistaken. In the small attic trying to locate the source of the whine is like trying to find a cricket--I can't really tell where its coming from.)