he method that works for me is to twist things around until I find an action that can reliably produce the noise. Then I get my son to do the action while I listen and feel around on the frame for the loudest spot.
Good checklist - thanks Jur! I'll print it out and go over everything.. The thing that reliably seems to produce the sound (when not on the bike) is to lift it up by the handlebars & saddle, then place it back down and press down on the saddle. It sounds like the noise comes from the front - but I should get a second person to do it so that I can listen more closely.