Smell in 2002 Honda Civic

We bought our 2002 Honda Civic new and have maintained it exactly as specified. Intermittently when we turn on the AC/heater we get a horrid smell, sometimes like dirty socks, sometimes like vomit, sometimes like damp basement. We recently had the cabin air filter changed. It doesn’t happen every time we turn the AC/heater on, but enough that it can be very irritating! Help?

Time to blow out the AC drain valve!

Whoa! Hadn’t had anyone recommend that! I’ll see if I can get that done. Thanks!