[MRCboicgy]

I'd go down to the furnace and steal a little pin that holds the cover to the furnace filter in place. Every time the heat comes on, there'd be a little tick/hum from the vibration of the filter that comes up thru the vents that is just loud enough to hear over the tv on regular volume and echo-ey enough during the night to drive you batty


source: one fell out of our furnace this winter like this