If a furnace is making whistling noises, there are several probable causes. While some of the reasons are simple in nature and can be fixed by the homeowner, some of the other potential sources of the noise will require attention from a professional service contractor.
Take a look at the hardware on the furnace for loose mountings or screws. Wait for the furnace to start running and then listen carefully. Turn the furnace off and allow the furnace to cool before tightening any loose hardware.
A dirty filter can reduce airflow, causing a whistling sound when air passes through the filter. Once the furnace is turned off and cool, open the main access panel to reach the filter. Remove the filter and read it to find out if the filter can be washed (permanent) or should be replaced (disposable). Place the clean or new filter in the furnace, close the panel, and then turn the furnace back on.
Once the filter is clean and the hardware is checked, look at the air return supply vents in the home. Check for blockages such as insulation, toys or any other object or material that is obstructing the vent and rattling and whistling when the furnace is running.
If the gas supply valve is defective or the burner chambers are dirty, the gas may make a whistling noise when passing through the system. A professional should be contacted to inspect and clean the furnace or replace the gas valve if necessary. Stop immediately if you smell gas when checking the furnace; turn the furnace off and leave the home. Call your gas company for assistance.