I'm really thinking a bird. I had a bird trapped in my cavity wall. I woke up one morning, youngest beside me in the bed, to sounds of scratching and ... noises... which sounded utterly like some scurrying-type creature, and so close by. I was utterly sure it was rattus rattus and have never been as terrified.
I eventually got out of bed in terror, and could see nothing, whereupon I realised it was something stuck (by this point) behind my fitted wardrobes. I assumed still it was a rat.
A while later, it sounded somewhat more bird-like, and eventually, through a tiny gap, I saw what I was pretty sure was a bird's little foot but still wasn't sure.
I managed to get a pest controller (weekend so it was expensive) for early the next day, by which time the squeaking / movement had stopped.
He cut into my cavity wall and could find nothing but was certain it was a small bird - they can fall into gaps from the roof / attic space outside apparently.
You are very good OP, as honestly, I did not care (well maybe a little) about the welfare of this creature. I was utterly unnerved, and just wanted it sorted. Of course it would have been good if it had lived, but I wasn't that worried about its death, and the exterminator man reassured me it wouldn't smell (it didn't).
I would say that's what you have. In terms of addressing it, yes, I'd call a private firm, but if they try to get to it, they'll have to take up your floorboards, yes - I still have the cut out hole, that I very badly repaired and at some point will get replastered - but I would definitely do it for peace of mind and to make sure that if it is a rat, it gets sorted.