TK device, TPU.. Only enter if you seek truth. Cause here it is...

[verpies]

The most characteristic feature of the sound you mentioned at 24m30s is its high- frequency content.
By "high-frequency" I mean >50Hz, e.g. 100Hz, 150Hz, 200Hz, which is not "high" by electronic standards at all.
The 50Hz buzzing is the strongest frequency that can be heard before and after the jack is pulled, thus it can be assumed that 50Hz is the main stimulating frequency.
Additionally higher frequencies can be heard which are most likely harmonics of the 50Hz.

This resembles a sound that a saturated transformer makes while an excessive current flows through its windings.  Welders often hear a similar sound. The nonlinearity of the saturated core can create higher harmonics, that can be identified in that sound.  This principle is used in ferromagnetic frequency multipliers (FFM). You can read about them here.

I am not claiming that this is a sound of a deliberate FFM. It is much more likely that it is an inadvertent FFM created by an overloaded transformer.

The transformer could be the one contained inside the DC/AC inverter you mentioned or another transformer that we do not see. (e.g. the "spool transformer').
A regulating circuit in an ordinary DC/AC inverter will respond with an increased current draw at its input to a falling input voltage. Possibly overloading and saturating its internal transformer.  I have no supporting evidence that it is the inverter's transformer that makes that sound.

Another interesting question is what electronic phenomena are capable of making a sound at all? 
Electricity generally is silent. So are transistors, diodes, thyristors, capacitors, resistors and integrated circuits.  High voltage can ionize air and produce hissing sound. Rapid ionization will produce a cracking sound of a spark. Magnetostriction will produce sound in transformers' cores.  Ampere forces will produce sound too, if wires conducting high current are allowed to vibrate.  Motors can make acoustic noise the same way as transformers do in addition to windage and bearing noises. Relays can be noisy, too.

That's about it, and this would tend to point to some kind of a strong alternating current in a transformer, as the source of the noise at 24m30s.

[elementSix]

Those Black Boxes both have heatsink fins on them.  So they would have to have something like Thyristors in them or the like.

[verpies]

Those Black Boxes both have heatsink fins on them.  So they would have to have something like Thyristors in them or the like.

Aren't those mounting flanges?
Thyristors or transistors have more than 2 terminals (wires).

[elementSix]

I read one comment, thats all I could find on the Black Boxes.  It it said that they are Chokes.  I don't know if it's right but I am reading a lot on the subject and other forms of Chokes and I found some useful info that pertains to our subject..  Will post more when I find what I am lookin fer...  What do you know about Harmonic reactors?? Or SMPS.. The switch mode power supply's..

[verpies]

Yes there is a great body of knowledge about SMPS out there. They are well understood and behave conventionally if NMR is disregarded.
The most common misconception about SMPS relates to flyback transformers. Most people drive them wrongly - like regular transformers, without a diode at the secondary, where the current in the primary flows at the same time with the current in the secondary (without an intermediate magnetic storage period). Read more here.