My neighbour wrote us a very polite note to tell us in this situation. I went downstairs the next day to apologise for the noise. I had no idea me chasing my new puppy would make any thudding sounds as I couldn't hear it from mine.
It may be a subwoofer if it’s a continuous sound like that. Definitely write a polite note! Do you have a property management company or perhaps the same landlord? If you’re unable to have a discussion with them, you may need to talk to your landlord.
Thump back. Or do what I did and walk upstairs and knock on their door in my postpartum underwear a button up shirt that had obvious boob leaks messy hair and hospital grippy socks and said “hi I have a new baby if you can’t tell *gestures to all of me* can I have you number to let you know if your being to loud or if you could just keep it down after *bedtime* because if I don’t get some sleep I’m gunna freak out…” they apologized and I didn’t hear any thumping after that (I was violently sleep deprived and had a tad bit of postpartum depression along with being petty af😅)