Does Agatha Heterodyne whistle while she works?
Here's one of the portrayals of Agatha heterodyning, from 6/6/07 (Volume 7):
Very cool image, but I'm having trouble figuring out precisely what's going on! Is Agatha actually singing? The speech bubble indicates yes, but what about the ghostly music that seems to be floating around the machine - is the music coming from elsewhere too? If so - where? Krosp says she "does that all the time" - does that mean that whenever Agatha is working, she's singing along, too? In other words, just what is Heterodyning, anyway?
Here's the earliest reference (AFAIK), from Volume 1:
But it's obviously very broad, and it seems contradictory - Agatha knows that the music can be heard ("Listen!"), but she also describes it as "making noises in her head".
In Volume 6, we also see Agatha get her clanks to heterodyne for her - which might imply that anybody can heterodyne (that is, produce the music), though it seems only actual Heterodyne sparks will actually benefit from it.
In short: What is heterodyning? Is it audible music, and does it require Agatha to be singing? Could other sparks heterodyne as well, or is this a magical family-only ability?