It seems it's when I'm slightly out the saddle as well, having just tested it. Also it's there no matter what I'm wearing.
Appreciate all the ideas alot, and although I haven't solved it... Yet... It's "nice" to know other people have gone through the same madness. Fucking Enigma.
Only seated you say ! I had a zip that used to tap on the front of my saddle ! Could not fathom out where the noise was coming from for ages !