That sounds like a ground loop alright. Does it change in pitch/intensity along with engine revs?

My guess is inductance from the various accessories are feeding a ground loop current into the head unit.

The sound probably isn't coming from the electronics themselves (i.e. coil whine), but it's inducing noise in the HU. It goes away when the HU is disconnected because nothing is driving the speakers.

Exacerbated by the high degree of integration nowadays with the infotainment system. It's never truly "off" anymore.


Previous threads (admittedly not very helpful):

Is this grounded sound system
Static Sound Sa Speaker

This last one made some progress by playing around installing ground wires:

Amp Hiss (even when RCAs unplugged; disappears when sound is played)