Arrived on site to diagnose a musty smell in home on a 4 month old system. Condenser located in back yard. Furnace and evaporator coil located in attic. Spoke to client upon arrival and stated they have been using system a lot especially since the heat wave started and they started to notice a musty smell in home recently when system is running. They stated they have been running system all day since it’s been hot and they run system from 72-74 due in the day and 69 at night so system has been running a lot. Advised I would be inspecting indoor unit to ensure what is causing smell. Inspected indoor unit and found filter hammered as they also use fan mode to circulate mode all day which is filtering home a lot more. Coil is good some light debris but nothing major. Advised filter should be getting change out every 6 months or as needed to prevent air flow restrictions and reduce the amount of dirt entering system. Advised if filter is not change out dirt and debris can enter coil and