Even with a dehumidifier and air conditioning our house has a musty smell to it. Don't notice the problem when the windows are open or in the winter when the heat is running. Had our ducts cleaned out hoping that would help but the smell is no better. House is all frame and is 31 years old. This has only been a problem for the last 2 summers. Have original furnace and AC units.
The smell reminds me of elderly peoples homes that are no longer maintained on a regular basis.