I have the same issue. I have the Harman Kardon AVI200. Recently it started making a loud buzzing noise, regardless of inputs or volume. I had everything disconnected except the speaker wire and one speaker and it still occurred. I did not try connecting it into a different power source. I just dropped it off at a local repair shop, but haven't received word back on the status. I'm interested to know if you have resolved the issue, or if anyone else has a suggestion as to the problem.