I had one that drove me to distraction (a squeaky-tick, that is - not a butternut squash risotto). Came from the front wheel area, and only seemed to squeak once per rotation of the chainwheel, sounding like it was on the right-hand side (from my POV whilst in the saddle).
Tried everything. Regreased the bars, the bolts, cleaned out and regreased the bearings on the headset, cleaned and regreased and readjusted the hub bearings, lubed the spokes where they crossed each other...
Turned out it was one of the springs in the suspension forks; drizzled lube down past the rubber boots so it went into the forks and that solved the problem.
So - if you have suspension forks, you might want to check them out too.
And check the little rubber 'cone' which covers the hub cones on each side of the front wheel. One of mine once came a bit loose and was squeaking against the fork as the wheel rotated.