Are you using a guitar amp or something? If it has distortion on, that could have caused it.
I would recommend using speakers or monitors regardless though. Unbalanced cables can also cause noise that like, especially if there is a lot of RF/interference.
New Recording Setup: iMac 2011/ULN8/Profire 2626/Axiom61Pro
Experience with the following M-Audio products: Profire series/Projectmix IO/Firewire Family series(410, 1814, etc)/Axiom series/Axiom Pro series/Oxygen series/Fast Track series/+more