The refrigerator thread reminded me of an issue I've had for years with my central air ducting. This is what originally led me to wearing ear plugs at night because it kept me up all night. With the weather changing this is going to be a problem again.
The main trunk of the ducting system is in the attic. There are branches which come of at various places, then flexible ducts which eventually make it to the ceiling registers. The air return box opening is in the hallway outside our bedrooms.
The problem is there is a bunch of ticking noise coming from the registers when the forced hot air starts and when it stops. When the gas furnace blows hot air through the ducts it seems like the main trunk and branches must expand somewhat then contract when the forced hot air stops. I don't know if the ducts are rubbing against the wood they're setting on or if it's something else. I've tried isolating sections by placing foam pads between the ducting and whatever structural member they're on but that hasn't helped at all.
I've checked the gas furnace to ensure the igniter isn't constantly clicking but that's not it. The ticking starts out fast and eventually slows down and stops after ten minutes or so. It's quite annoying, especially at night when everything is quiet. I've gone in the attic and tried to identify the source of the sound during the day but the road outside my house drowns the noise and I can't hear it. It's only late at night when it seems so loud.
Any ideas?