(SOLVED) What is a whining, high pitch sound coming from my furnace when heat kicks in
It's unlikely that going from a larger pipe to a smaller pipe (which is essentially what the flex hose is) would cause that. However, the repair man may have meant that the flex hose was undersized for the gas line inside the furnace, which would make more sense. Expanding flow could certainly cause the noise, but to really figure out what's causing it you'll need a mechanic's stethoscope. They're pretty cheap, but you may even be able to get one as a loaner tool from an auto parts store.