Dry the place out. Opening up the windows and airing the place out can help.
If your house tends to be humid and you’re sure you don’t have any leaks, keep your air conditioner or a dehumidifier running.
Let the sunshine in. Light, especially sunlight, with its ultraviolet component, is a good disinfectant. Letting a little more light into an old house can do wonders for the musty odor and can help protect against mildew and mold.
Clean your couch. If that old house smell is coming from your couch, it may be more of an "old couch smell" instead. Upholstered furniture and your carpets could be culprits. Soft stuff absorbs ambient moisture, which can lead to mold growth. All the soft should go or be professionally deep-cleaned, along with a good cleaning of the hard surfaces with a disinfectant.
Deep-clean the guts of your house. Furnaces and air ducts can have a tremendous amount of mold that can grow in them when they’re not being used. Not to mention that air conditioners can also trap mold and mildew in their filtering systems.
If you think these little tunnels are the source of the odor in your home, hire an HVAC professional, who can actually use a tiny camera to make sure all the gunk is located and removed.
Declutter under every sink. Getting things out from under the sink lets you see if anything is dripping or molding. Leaks under a sink can definitely cause an unpleasant smell, and if you haven't inspected the area in a while, the musty smell may have gotten worse and worse until the whole house has an odor.
Wash your walls. If you've tried everything and still haven't found the source of the odor in your old house, the musty smell may very well be coming from the walls. Dissolve a half-cup of borax in a bucket of hot water (32 ounces), 2 cups distilled white vinegar, and 16 ounces of hydrogen peroxide and wipe your walls and down and let them air-dry.
Neutralize the air. An open container of baking soda or white vinegar, kept in an unobtrusive place (for example, on top of your kitchen cupboards), can help absorb musty smells and clear the air.
Experts also recommend FreshWave or DampRid, two all-natural substances that absorb smells and trap excess moisture in the air. These can help you get the musty smells out of your house—and get your place ready for company!
Information Provided by Stephanie Booth, Realtor.com