Like a good Mustachian, I had not been running the air conditioning. Upstairs, no problem, leave the windows open and the ceiling fans running and we got past Memorial Day.
But, the basement was closed up during that time and with no HVAC running, it got funky smelling. Within a week of my turning on the A/C system, the smell was much better. At this point in the season, with the A/C running 24/7 (it's Virginia, after all), there is no smell at all. Fortunately, I didn't buy a dehumidifier. The one I found on CL was already sold and before I could find another, I noticed that having the A/C on seemed to be helping.
Also, I have new tenants! They moved in July 11 -- a lovely newlywed couple in their early 20's. He's a music minister at a nearby church. She majored in classical piano and harp. Not only is the rent on time, but they're super nice and I enjoy free concerts from time to time. Hurray!