Pickups are what "pick up" the sound of the guitar strings and turn them into an electrical voltage.
Is it an electric guitar Mr. G? If so the pickups usually sit on top underneath the strings where you pick/strum the instrument. Single coil pickups are about the same width as your finger and run across under all the strings, the often have six pole pieces (the tops of magnets—basically chrome dots) that line up with each string. Humbucker pickups are designed to reduce hum. They are normally twice as wide as single coil pickups but sit under the strings in about the same place and do basically the same thing (convert the vibration into an electrical signal).
If Sarah @KMI is asking you to fix a grounding issue then yes, maybe a different guitar would help to work out if that is the problem. Do you have one that you can borrow from a friend?
I seem to remember though Mr. G that the humming didn't happen when the SoftStep was not plugged in ie. Motu+Guitar+Computer = No problem. If that's the case it seems strange to me that it would be a grounding issue with your guitar. If the problem was with the guitar I'd expect to hear it with just the Motu.
Also, is it possible for you to re-post the sound file of the noise? The free hosting of the file has expired.
As a prospective buyer I'm more than happy to try to help you through this issue if I can. I've been working with music making gear for a long time now so may be able to provide some insight into how to isolate the problem.