This may be covered somewhere, but frankly I can't even figure out where to start looking.
I have a 1998 Bianchi San Remo road bike, with original components (component details can be seen here:
1998 Bianchi San Remo - BikePedia - gearset, shifters, breaks, wheels, are all original). The bike is very low mileage - when I bought it a few years ago it still had manufacturer's grease on some sprocket wheels, and I've only put a few hundred miles on it. Only changes to the bike were seat, post, bars, stem, pedals, and tires/tubes.
This year was a late start on my biking season, so I took the bike to a local shop for an annual cleanup/tuneup, they that there were no issues. Wheels were inflated to 90psi before the ride.
When I took it out for the first time, there were funny noises from the rear of the bike. For all the world it sounded like a dragging brake (this bike has cantilever brakes), so I pulled over to check, but the brakes were aligned correctly, and I could not reproduce the noise off the bike. I stopped and checked several times - with no load both wheels spun completely freely.
I noticed the following: The noise is worse when I'm coasting than when I'm pedaling (pedaling mostly but not completely eliminates the noise). The noise is much worse if I deliberately shift my weight back. I'm heavy (~300lbs), but never had issues with this bike reacting poorly to my weight before.
Once I got home, I put the bike on a stand, but no matter what I did, I couldn't get the noise to come back.
Any ideas? I can't even figure out where to start looking for a problem...