EDIT: SOLVED. IT WAS THE FLEX CONNECTOR. THANKS EVERYONE. My gas furnace makes a high-pitched whining sound when the burners turn on (when I can see fire). A repair man came and told me it may be that the “flex connector hose” that goes from the gas line to the furnace. He said the flex connector is 1/4 inch when the gas line is 1/2, creating a bottle neck, stifling the system from getting enough gas. Could that possibly lead to a high pitch noise that is always present when burners are on and on occasions can be heard anywhere in my small home?
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.
I had the whistle in my last home ,, actually in my shop , the corrugated flex whistled when it was at full blast. Last try for tonight connect or ? Problems. I upsized from 3/4 to 1” flex and the whistle was gone, it’s the flow and the ripples that make the whistle, upsize the flex 1 size you double the area and noise gone, ok last time . ok this worked for me it made a huge difference , after doing this my hunting buddy said look at your elk call exactly the same blow two hard and the squeal scares them off, my shop heater was 125 or 25