I'm 16 weeks.
I started feeling rolling sensations at around Week 15, about 2 inches below my belly button. Sometimes it was rolling, other times it was like an internal nudge.
Now it sometimes feels more like solid kicks, but how you're describing it is certainly how it started. It's somewhere between a kick and a roll most of the time for me.
I didn't feel anything this pregnancy until 18+ weeks. I was surprised and worried since this is my 3rd baby 5th pregnancy. I was sure I would feel it much earlier. I've only felt the thumping a few times since. Ready to feel some kicks all the time!