Hi, Im trying to find the title of an old black and white movie that may be a Hitchcock or earlier. There's a murderer who kills a woman by turning on the gas and hiding underneath floorboards. Can't remember the name for the life of me.

I nicked the answer from another site:

Result: 10 Rillington Place (1971) starring Richard Attenborough, John Hurt and Judy Geeson. Lurid but excellent account of British serial killer John Christie. Thanks Patty. Jan 12, 2008

The person "nicked" the wrong answer!

The Snorkel (1958)
IMDb synopis:
Paul Decker murders his wife in her Italian villa by drugging her milk and asphyxiating her by gas. He cleverly locks the bedroom from the inside and hides inside a trapdoor in the floor until after the body is discovered by servants. He uses a scuba snorkel connected to tubes on the outside to breathe during the ordeal. Decker's stepdaughter Candy suspects him immediately, especially since no suicide note was found. She also is convinced that he murdered her father years before, but her accusations fall on deaf ears. The ruthless Decker even poisons the family spaniel when the pet takes too great an interest in the mask and realizes he will ultimately have to get rid of Candy, too.

The Snorkel (1958) - IMDb