My road bike developed a small 'squeak' that for all the world sounded like metal-on-metal scraping and coming from the rear. I had my wife ride along side of me for about 2 miles and she was pretty sure it was coming from the front. When I pulled the front and rear wheels, mounted each in the truing stand and tried to diagnose where the noise could be coming from. No grinding feeling--the hubs felt fine--and no noise at all.

Finally, after a local club ride, while I still had the bike out in the car anyway, I took it to the LBS. Instantly they knew what it was just from my description. The rubber boot around the front hub was a little dry and when mounted was just rubbing enough to cause the squeak. A little touch of oil solved the problem.

New diagnostic procedure: At the first sign of an unidentifiable squeak, dunk the whole bike in a vat of 10w-30.