Does he have to define "thin"? If the engineer was checking the house and he heard alot of noise from the neighbours house then I would be happy that he made it known to the potential purchaser. I would say that 90% of the time an engineer would just check for structual issues and not be bothered of he heard excesive noise or not from the adjoining property.
I know people who bought newly built houses and they have since moved becuase of the level of noise comming from the neighbours house and by noise we are talking about normal level conversation etc..We all know that all the new builds meet the building regulations but we also know that those regulations meant practically nothing over the last 5-10 years when houses were thrown together.
I would be happy that the engineer happened to be inspecting the house at a time when there was activity in the neighbouring house and that he actually bothered his This post will be deleted if not edited to remove bad language telling me. I would walk away.