Maybe it was an oil burner instead of a gas furnace. I remember it was a large furnace in the basement that had a strong low frequency rumble produced by the flame. The fact that the sub-sonic rumble modulated Steve's transmitter as much as it did says a lot about the low frequency response of his rig. And this was before the pulse-width modulator or class-E rigs. He was running a pair of 833As modulated by another pair (IIRC), using RCA 1-kw broadcast iron.