Car PSU or PC Based? Hum!

Hello all.

I didn't know where to ask this, but I have an annoying hum issue! I've got an early Pi installed in my van. I bought a Shenzen 5 volt switcher

https://www.amazon.co.uk/gp/product/B00HV4EPG8/ref=oh_aui_detailpage_o07_s00?ie=UTF8&psc=1
It supplies my pi and Muse USB dac

https://www.amazon.co.uk/SMO-Conver...&sr=1-2-catcorr&keywords=MUSE+USB+DAC+PCM2704

Well, I got this annoying hum. I've unplugged the rca composite video to lcd display, does nothing, unplugged the lcd display power, does nothing. Got a Lowpass filter on the main 12 volt line and elliminated the alternator whine I had, got de-coupling caps on the audio rca lead. The system is silent until I boot the PI, you can hear the text load, the desktop display, you can hear VLC player being launched. Seems like the noise is beeing fed from the PI through the USB dac!

So, I think I've dealt with the usual Ground Loops and alty whine, what else can I do?

Thanks, Mike