Hi,
""Unplugged laptop from charger: buzzing gets louder" hmmm... odd, but perhaps a clue here? If you have a powered hub, you might try that? The Audiobox USB (the original, not the iTwo) is a PORT SPECIFIC device, and MUST be used ONLY on the port it was installed in. Here is where it gets funky... USB ports are not all created equal...that said, some have more power available to them, and sometimes, changing ports can solve the problem. HOWEVER, if you want to try it... you'll need to uninstall, and re-install to a different port.
A Powered USB hub can solve the problem (if indeed power is the issue) and doesn't require a different port install...
This and more troubleshooting are available in the 'Knowledge Base' :
http://support.presonus.com/forums
The uninstall process, and other cool articles are available there...