The Case Of The Laughing Mummy is a Sherlock Holmes television movie from 1955.
It stars Ronald Howard as Holmes, H. Marion Crawford as Dr. Watson and Archie Duncan as Inspector Lestrade.
While on a train journey to take them fishing, Holmes and Watson meet an old friend of the latter.
He tells them about a mummy he has at home that breaks out into laughter and the two go off with him to investigate.
It turns out to be the noise of a weathervane but in the process Holmes works out that the mummy is in fact the remains of a modern man.
Movie pages here.