I saw a film many years ago, I think it was black and white. It was about a woman who is made to believe she is going mad. I think there is a man hiding in her house trying to convice her she is going crazy, I think he is hiding under a trap door in the floor. Anyway at the end of the film her friends convince her to leave the house and just before they go they move a piano across the floor trapping this man underneath. As they leave the lady says, can you hear the knocking. Please if anybody knows what it is called I would be so grateful.

Possibly 'Hush Hush Sweet Charlotte'? (just a guess... )

Sounds like The Snorkel.