Mm that's frustrating alright and if you took the headset and the cups off and the scratches on the frame don't seem very deep and run parallel with the tubing then it's certainly a mystery because the tubing having a very fine split running just a short distance along the tubing will allow the tubing to actually twist very slightly under pressure then ping when the pressure it taken off but seen as you can't find any small splits or cracks then i dunno what to suggest ..
