I have a boiler type of furnace in my home (AO Smith, Model# HW120P720, photo attached). When it gets colder outside the boiler makes a "whining" noise when the gas/pilot comes on to heat up the tank. The noise stops as soon as the gas shuts off. Is this a gas regulator that is going bad? If so, could it go bad quickly altogether and effectively shut down the boiler? What do they cost and how hard are they to replace?

P.S. Don't have "HVAC 1000" answer this question as he appears to be clueless and rude. I have verified this model number on the unit THREE TIMES.

That boiler is obsolete and now I recognise the unit from the picture. Those were actually meant as a swimming pool heater or a water heater with a stack tank.

When you do this as requested in my original post you will find your problem. It is when people do not follow directions when we are trying to help that causes the problems. NOW do what is posted below and report back.

I show no AO Smith boiler with that model number but here is what you can do to isloate the problem. Use a non conductive device and place it on the suspected component on your boiler ie the pressure regulator/gas valve. Then place your ear to the device and have someone turn the boiler on. This way you can hear and isloate the problem and make the repairs if necesary.

Since it will be difficult to locate some of those parts I can help in that issue. I do not have the old parts but I know a laundry mat owner that probably does.

That is a old Burkkay commercial design hot water heater with a AO Smith label. Just talked to my friend in the laundry biz. He sent a file with the part numbers. He said that the coil inside gets limed up and causes noises on his units and he has them taken apart and a acid solution circulated through them to clean out the lime deposits. Still use the test above to really find the problem before you tear it apart to clean. PS never bite the hand that feeds you.
Some parts still available at www.hotwater.com

Good luck