Here's a delightfully weird little thing:
On my new Windows computer, I played around with Dragon NaturallySpeaking (dictation software), and with Win7's built-in dictation feature.
At some point, the computer started making a sound-feedback, and an infernally loud screech developed within seconds. In desperation I plugged in a headset to stop it.
And now I can't unplug the headset without the feedback screech developing again! This is despite the speech recognition now being turned off, and the machine having been rebooted...
Update: Robb said:
You have an open microphone somewhere that's feeding back into the Dragon software. Kill (mute) the microphone and the feedback will disappear.
But Dragon is off. And the only mike is the one built into the iMac.
(I used a USB mic when testing Dragon, but that's unplugged.)
But you could be right though I don't understand it: I just now went and made a search for 'microphone' and found an "audio device" control panel. There were two active microphones in the panel, which I guess must be software items, not real things. I "deactivated" the one which seemed related to Dragon, and the problem is now gone. Thanks.