Recently swapped out my Kona LHe for an I/O HD. As per AJA's advice, I removed the old drivers and software/control panel before installing the new software.
I can't explain what causes it, but if I switch back and forthe between apps, any sound (iTunes, QT, Internet source, FCP, etc.) starts to echo on a 1 second delay and create a feedback loop. Sometimes switching apps again or causing another audio source to begin will break the loop. Sometimes I can only stop the source that's feeding back and restart software/hardware.
This is a bug with the IO HD, go to sound prefs on the mac and deselect IO HD out and select mac output. The io HD will still handle the audio out so no worries. Have a search on this forum and you should turn up something.