I still think they’re picking up ultra-low frequency EM radiation. Not enough that you could power a car, mind you; just enough to show that there is a signal… say, in ~ 7.5 to 15 Hz range, and harmonics. With good antenna and a high Q resonator, you should be able to get a few microwatts — more than enough to register on an oscilloscope.
Such radiation could originate from ionospheric interactions of solar wind and earth’s magnetic field.