Thanks for all the replies.
I've got a 3.5mm minijack feed coming out of the back of the stereo, that's the AUX, going into a dual splitter lead 3.5mm and lightning connector, then into an iPhone. So the USB is just power, not data, between the phone and the USB socket.
When you pull the USB out of the charger, the noise goes away.
The 12V socket is cheapo, but it was the smallest one I could find that was 12V cigarette and twin USB sockets, I shouldn't have expected much for £3.99!
What's the AUX, is it what you have plugged in to the USB charger? Does it still do it if it's not being charged?
You could try moving the 12v wiring. Put the earth on your radios earth. twist the + & - wires together, like twisted pair ethernet cable.
Odds on it's the cheapo DC DC converter - yes ferrites are worth a go, try 'em on the 12v side first.