Dear John
Thank you very much for your advice. I did give it a go with and without mains power and it didn't seem to make a difference. However, I must apologise and should recharacterise the noise being recorded as really more a "shhhhhhhhh" type sound rather than a "hum" per se. With the USB mic combo plugged in, but resting idle, there is one green bar on the VU Meter Input bar in Realband. Any other thoughts?

Thank you for your time.

Regards
Zoë