When I stepped into the hall of this 1900s house I felt the floor move slightly beneath my feet.
Pulling back the carpet revealed small flight holes from wood boring beetle infestation (“woodworm”), and importantly from the surveyor’s and buyer’s viewpoints, little piles of sawdust (known as frass). This indicates recent activity.
Floor boards need lifting to inspect the supporting joists below. Badly infested timbers should be replaced and the remainder treated with a proprietary insecticide by a competent timber contractor in accordance with the manufacturer’s recommendations.
You should obtain a comprehensive insurance backed guarantee for such work.