Hi,
I'm using aircraft power and one of these as per the link.
https://www.amazon.co.uk/Anker-Charger-Adapter-PowerDrive-Samsung/dp/B00VH84L5E.
Its great that you don't get any noise, I've tried the audio jack that's supplied with the BOSE A20 and I have also tried a different cable like the one you would use to connect sound card from a PC to a the speakers within a monitor and get the same result.
I don't have any low down noise if I plug a mobile device in, only the Raspberry PI boards.
I'm running the latest PAW software too, I've even tried a new Raspberry board with nothing installed and once powered get this same low down noise coming through. It goes quite if you touch any USB, LAN, HDMI part thus earthing them.
It's like hissing sound, it's low in volume but you can here it, until the board is earthed.
I see you made your cable, I was looking at doing the Bluetooth connection, but like the idea of keeping that free in case I fancy some music as the AUX connection gets priority
done some testing and works well