As a pilot, we also get annoyed by the squealing in our headsets by those pilots who use the speakers + mikes.
I think the problem might be that pilots aren't checking that the intercom (selected on the transmit/int button) selection is actually off - this tends to cause feedback. Well, that's what happens on the 146 anyway.
Hope this helps !!
[This message has been edited by Gonegrey (edited 27 June 2001).]