Our furnace started to whistle. Not a tune, but just a steady whistle when the blower came on. It kind of wavered in pitch a bit, but it was not going to win any contests for being a catchy tune. It was rather quite annoying. We could hear it all over the house, but it was louder in the bathroom downstairs. I called a Sacramento HVAC specialist to come out and take a look. We had not been keeping up with our annual furnace cleanings and inspections. I have been keeping the filters changed and the electrostatic filter clean. It sounded like the whistling was in the ductwork.
The technician took a look at our system and figured it was a problem with the zone control system. You can choose to heat one part of the house hotter or cooler than the other part. The zone system uses dampeners in the ducts to control airflow when the system is on. We can make it warmer in the living room while we are watching TV, and the kitchen can be cooler where the cooking is going on. The bedrooms are another zone. We keep it cooler in our room at night, but the kids like their rooms warmer.
The whistling was coming from a stuck vent damper close to the bathroom. It was controlled by a servo motor to open and close. It was stuck in a position where the air blowing over it was making a whistling sound. The technician was able to fix it after getting access to the duct through the service panel in the laundry room. I would have never figured that out even though I was very familiar with how our home heating system worked. I was glad they sent out an experienced technician to fix our HVAC system.