Performed customer 2 system fall (HEAT) homecare maintenance check Upon arrival customer stated that when ac is running for both systems that they both were having a musty smell to them. Did full evaluation for the attic system. This is a due fuel system. Gas furnace is 9 years old, ac and coil is 1 years old. Changed (14x20x1) system filter. Turned ac on and after about 5 minutes of run time the unit did start to smell. Unit is affected by “dirty sock syndrome” which is bacteria growing on evaporator coil at the indoor system. Turned system to heat and no issues. Checked over all operation of system, blower wheel is dirty, cleaned flame sensor, cleaned pressure switch tube port. Checked amps on blower motor and inducer draft motor and found no issues. Inside system is clean. Checked outdoor heat pump, capacitor and contactor tested good, condenser coils are clean. Recommend blower wheel cleaning, dirty sock syndrome cleanings for evaporator coil and adding Uv light to system. Custome