I recently got a new furnace system and the brand new device started making a whistling sound. I thought that was how furnace devices operated until my brother complained about it. He told me that a heating system making a noise meant something bad. Since the device was brand new, I was really angry at the thought of it already being messed up. I looked online and found that a clogged furnace filter could be the reason for the whistle noise. The reason the sound happens is that the airflow is being stopped in the system. The air filter could be the cause because if it gets super caked on with dust, no air can get to the system. So the furnace has to find a way to move air in a different location and this causes a whistle. I checked the HVAC filter and it looked fine. Even though it seemed clean, I did buy a whole new one and that did not fix the issue. I then looked into other possible issues and one was that there is a gap in the ductwork. A small hole between the duct and the furnace’s blower will cause a loud sound. I looked all around in my furnace system and even had my brother take a peak, we could not find a gap. I finally called a HVAC guy into the home to see if we missed something. The guy right away said the issue was that my ducts were actually too small for the furnace device. The furnace needs larger ducts for greater airflow. So that was an expensive appointment.