ANSWER: Very dirty furnace filter
Filters in HVAC units, including heat pumps, AC units, furnaces, window units, as well as larger units, are used to keep indoor air cleaner and HVAC units operating more efficiently. Keeping a clean filter in your unit can also extend the life of the unit, as it will not be working as hard. The more dirt and dust the filter collects from the air, the less dirt and dust pass through the unit and back into the indoor area. Filters should be checked every 30 days and changed every 3 months maximum to keep your unit operating at maximum efficiency.