I was making preparations to receive the SAQ VLF transmission on 17.2 kHz.
I noticed high levels of noise at VLF when used with some USB power sources.
The remedy is to add a 1000uF capacitor between the 5V input from the USB socket and ground on the base unit.
This completely clears any background noise from any 5V supply I've tested so it's a recommended modification for all units.
The photo shows where to wire a 1000 uF capacitor (lead lengths are not critical).