I have a Sennheiser ME 66 and I'm using an H4n to record. I'm new to recording, and it's new equipment. I was recording someone playing guitar and while their voice was perfectly clear while singing, the guitar made a strange "thump" sound every time she strummed, or even when she picked with her fingers. What is that called? Can I make it sound better right out of the microphone? What should I be looking for in a sound editing software that can get rid of that?

I did as much research as I could, but "weird thump" doesn't seem to get a lot of hits.