Moisture issues.

I bought a house that was built in the 1920s. Washington state within 10 minutes from the puget sound. Lathe and plaster interior and cedar/wood shingle exterior. I noticed intermittently there is a moisture/mildew smell present. Especially when it rains in the warmer months. My wife asked me to buy a dehumidifier the other week and it pulls about a cup or more of water from the air. The obvious verdict is moisture issues. The house appears to be in fantastic shape for 100 years old and the roof doesn’t leak. I don’t have any moisture patches or water damage in the plaster that I can see. What kind of service do I call to find the root cause of the moisture? Or at least where do I start?