Strange noise from under floorboards

[Fleurchamp]

And before anyone says it - it is not mice/rats - we have had mice before and it is not the same noise.

We live in a Victorian terrace (currently selling, exchange imminent) but over the past week DH and I have heard a strange noise coming from the corner of our bedroom. It is a kind of rustling/ creaking sound, not very loud (we mainly hear it when it is quiet at night but I have heard it during the day too).
The floor is just floorboards but we don't want to start pulling them up because they are the originals and prone to breaking.
It sounds like some kind of insect, too light to be a mouse, but I haven't been able to work it out from googling.

I would hate to exchange/ complete on the sale with this issue in place. I would rather know what it is before disclosing to the buyers and we would get someone in to treat before completion.

Any ideas? The room isn't damp.

May be little birds in external brickwork? I've had rustling sound at similar times during the year and pest control found nothing in the house but I could see the birds popping out from under eaves, in between bricks

The pest guy came - more floorboards came up (one snapping ) and he found nothing - no evidence of any woodworm or anything else. I haven't heard the noise again either.
Such a mystery! I think I am going mad

Poltergeist?

it's worth recording the noise on your phone, and in different spots to see where it is loudest. A pest person might recognise it.

even wasps rasp at wood.