This is a tried and tested method for proving a noise issue. The problem only arises when you are recording conversations without the person’s knowledge and then try to use that as evidence in a court case or in some other way that impacts on them (like putting it on YouTube or Facebook).
I only suggested sending them a note alerting them to the fact that their noise would be recorded as a way of sending them a warning without it being an obvious threat.
As the examples on here have shown, you can do it and use it as evidence and if you’ve sent them a note telling them that you plan to record their thumps rather than their chats, they really have no comeback now or when it goes to court.