All...night...long.
My son does this thing at night where he raises both of his legs at the same time, REALLY high and then slams them down. Since he's in a pnp bassinet it's really loud. One night he did it so loud I thought someone had broken in. I'll wake up, ready to start a feed/change and he'll be sleeping, and continue to THUNK in his sleep. I'm not sure what to do, we thought at first it was because he hated his swaddlers so we started putting him in pajamas. He definitely likes the pajamas, but it didn't stop the thunking. We could move him to his crib in a separate room but we're worried about the SIDS risk because he's only 15 weeks!
Re: THUNK
ETA she has never been swaddled (hated it) and sleeps in just a sleeper, sometimes with a sleep sack if it's chilly. She does it no matter what she's wearing.
And yes, it's normal for babies to do that. I usually hold his legs down till he's asleep. It's all just them playing with their bodies.