Bill, sometimes the kb harmonizers, particularly the Yamahas, pick up extraneous sounds, such as from the speakers, and try to "harmonize" those too. It usually results in something such as what happened here. Unfortunately when you play live it does happen sometimes. It's generally on a loud note when you back off the mic a little, allowing other sounds in. I've found the problem much diminished since I got the Bose PAS.
DonM