When you make a rhythmic strumming sound, either with your voice or a stringed instrument, you thrum. If you and your friends all play the guitar, it may be fun sometimes to sit around and thrum together, improvising tunes.
This word can also be used as a noun, to describe the kind of melodic hum that guitar strings make when you strum them. So you might say you found your way back to the campsite by following the thrum of your mom's mandolin, or that you love the thrum of rain on the metal roof of your grandparents' house. Thrum, like strum and hum, is imitative, mimicking the sound it describes.