Aw poor bun!  Archie will do this every so often too.  Even if there is no noise and we look outside and don’t see anything, he’ll look terrified!  Rabbits have very good hearing and your’s might have heard something outside, maybe another animal.  I usually just pet him and tell him everything is ok and offer him a treat to calm him down. 

They do that sometimes. When mine thump (periodically thru out the day… sometimes several times), I just check them and make sure everyone seems to be okay. It’s a signal of danger, but I think in domestic buns it can mean displeasure at something: a break in schedule, a loud noise, strange smell…

My husband has an (annoying) ringtone on his phone and anytime it goes off we’ll hear a cascade of thumps…

I often wonder if bunnies can see ghosts! Mine does this sometimes out of nowhere. No big deal. Just give her lots of love.

My bun hasn’t done this yet, but my gerbils do it every so often. I think sometimes they’re in their on little world and not paying attention, then get startled when someone comes into the room, makes a noise, etc. I haven’t figured out any way to avoid it! Sometimes they’re hearing outside noises that we can’t hear at all, so it seems like they’re getting upset for no reason.

Maryann can detect earthquakes so when she’s thumping for no reason, I make sure nothing is tottering.

Dono has thumped for no reason from time to tme. Well the reasons I’ve believed have been anything from opening a soda bottle to hearing the beeps of my A/C coming on/turning off.

my little bunbun does it all the time and to me she seems to be playing because she will thump and run across the run just to thump again and run back. she will go back and forth. if she thump because she is scared she comes to me, so all other times i just check on her then let her play.

I have found that so many times my listening filter is on – like noises going by the house, like dog tags rustling as a person walks their dog on the sidewalk in front of the house, or if the neighbor gets in and out of his car – the door opening and closing. All kinds of stuff that I am not really aware of until I hear them thumping. Sometimes it’s much quieter, like a new strange sound from a fan, or something on the radio.

Just try and listen carefully and see if there is anything you can detect. Does it seem to be around the same time?