Ghosts in the Speakers

I’ve got two sets of D’Feldt Audioengine 5 “active” speakers (i.e., with built-in amplifiers).

Yesterday one of the sets (in the office) started making a thunder-ish noise, but only in the “sattellite” speaker.  So I disconnected it.

This morning I was awoken by an insane noise.  It sounded like someone were putting asphalt down in my apartment.

It turned out to be the other set of the speakers, in the bathroom, was making the racket.  It sounds kinda like the noise in the other set, only louder.

The sound is very much like the sound that broken amplifiers make when warming up.  Just listen:

See?

There’s nothing connected to these speakers except electricity.  There’s no audio signal.

What are the odds that two pairs of these go bad — this bad — one day apart?

It sort of seems not very likely.

Does anybody have any other explanation for this phenomenon?  Animal magnetism?  Comic rays?  Neighbors conducting experiments with their home-built hadron collider?