I'd pull the cranks and spin the bearings with my fingertips to check for any crunchiness. This could be debris or damage in one of the bearings. The Gossamer MegaExo bearings aren't very well sealed and tend to get crunchy easily, at least on CX bikes. On the family fleet I use a Raceface bearing press to pop the old ones out of the cups and replace them with Enduro bearings which have held up much better.