is the smell mildewy or old house? then there is probably some mold somewhere

Have a pro come in to check for moisture in the walls. Or dead animals.

Thanks all. Everything about the house is in good repair (we had a 4 hour home inspection before buying), the smell is that old one you get sometimes in old wood furniture.

Have you had the heat and ac vents cleaned? Old pet dander, hair, etc can can get caught in the HVAC ducts causing it to stink.

Do you any carpet? We ripped out ALL 15 year old carpet in the house we just bought....it made a huge difference in the smell, as well as my son's allergies.

Try running a dehumidifier continuously for a couple weeks, then as needed. It will take away a lot of odors. Also, if there is a dirt floor in the basement, you will need to pour a concrete floor there to get rid of that smell.

Don't forget to check the attic. The old wood up there has decades of dirt on it. In an old house we once had, we painted the entire attic in order to seal up all the wood. Febreeze works wonders up there too.

I have narrowed the search down to 80 year old wood in the basement stairwell. I did get rid of the smell in the mudroom cupboards by sweeping them out and then spraying the wood with eucalyptus essential oil and water. It worked really well so will be heading down those stairs with a vacuum cleaner and spray bottle later this week. Also will be opening any windows downstairs and the basement door as soon as the weather looks right. Thanks.

Cedar oil in spray pump bottles ($10) can be useful in some old wood situations. It does remind me of hamster bedding, though.

Baking soda worked for us in a home we renovated that had very strong cat urine smells throughout . Give it a shot.

I was recently told that ceilings hold quite a bit of odor, so paint the ceilings as well as the walls