Recently I just got a brand new furnace, condenser, coil, the works. Everything works great except for a whistling noise when the unit is continuously off for a few hours. The HVAC professional who installed the unit said that I need another return vent installed as there was not enough air flow into the blower with this brand new unit. He came across this solution because when he removed the bottom door covering the blower, which allowed for my air to enter the unit, the whistling went away. Since another regular sized intake was installed the same issue has followed to my disappointment. On hot days when the unit shuts on and off throughout the day, the whistling never comes on. Only surfaces when unit doesn't run for about 8+ hours. I feel there is some sort of pressure that builds up causing whistling. I have a 16x15x5 sized filter in there but I ran system without filter in and whistling still surfaced. Any ideas would be great.