If I unplug the USB cable, the noise immediately goes away. As the IO interface is a guitar effects pedal I can use the pedal with the same output connection and the sound is clean and without intrference. It's only when the USB cable is plugged in.
Is this the fault of the cable? I would have thought that as USB was digital I shouldn't be able to get interference noise like this.
I unplugged the IO device from the Dell laptop I was intending to do my work on and plugged it into an older tower PC. The difference is immediate - virtually silent.
It seems to be something to do with the USB ports on the laptop.