Radio Noise from Storm
Assume 100 hertz for the growling. Or 10 millisecond period. Or 10,000 us period.

Electricity travels 1 mile in 5 microseconds. thus this was a 2,000 mile event.

Could be the local charges getting replaced, from 2,000 miles way.

Lightning requires cosmic-ray energy; approx. 70 amps per second worldwide average current from outer space, per Feynman.

That current needs to make its way, from all around the world, to the various storms dumping change down onto the ground.

Thus, if this is correct, your storm was getting replenished from 2,000 miles away.