What is the movie opening theme that goes bum DA DUM and then some timpani drum beats repeat?

The movie opening theme you are describing is most likely the 20th Century Fox fanfare. It begins with a low, sustained brass note (bum DA DUM) followed by a series of quick timpani drum beats. The fanfare was originally composed by Alfred Newman in 1933 and has been used as the opening theme for most 20th Century Fox films since then.