My vote is for debris trapped in the tire carcass. Seriously. It recently took me AND my wife, nearly 30 minutes to find the little shard of glass that was embedded IN the tread of our Bontrager Hardcase (H2) on our tandem. Those tires are damn near flatproof but when something does get in them it really does get IN them and you need surgical equipment to get it out. Get one of those headlamp mounts for your bike light and light up the inside of that tire. Glass or tiny metal wires will glint in the beam. Flex the tire as you go around carefully feeling along. Investigate EVERY break in the smooth lining of the tire. It's in there.