Performed preventative maintenance for gas furnace. Inspected all the electrical components. Confirmed proper high and low voltages. Cleaned the burners, flame sensor and burner compartment. Replaced the customer supplied Merv 11 20x25x5 air filter. Tested the amp draw for the blower and inducer. Both were operating within the manufacturer specs. System had a temperature rise of 53°. Inspected venting for restrictions. Performed a combustion analysis on the furnace. System is operating at 83.3% efficiency with a CO level of 12ppm. System is operating properly at this time. No charge due to free with install maintenance. Found whistling noise seemed to come from the dirty filter. Once changed I found the noise was gone. If you hear the noise again give us a call and we can further investigate. Model #SL280UH110V60C-05 Serial #5921D17149