And what are you & Reynaud imbibing?
   
   Yes, it's possible that you are hearing overtones. However, when I am playing whistle along with an accordion or concertina player, it sometimes sounds like there are extra reeds which have air moving through them, and indeed, that can happen. SOmetimes the reeds are just "sticky" or not quite right, and a reed or two will sound when it is not supposed to. It's usually the same one or ones every time.