You know, that music they play in really bad movies to let you know that Something Bad Is About To Happen…
Does it have a name? Is there an MP3 of it somewhere online?
Well, this thread will certainly get a lot of views (and possibly a prima facie moving to El Pit 
I believe the traditional way of typing out the music you’re refererring to is
“Dun Dun DAAH!!”. No idea what the name is- “That music from TV and movies when someone finds out a terrible secret”- seems to work as well as any. 
That three-note music cue, played on horns, goes back to The Creature from the Black Lagoon (1954), whose score had no credited composer. The three-note cue was titled the “Creature theme,” and was heard many times during the score, and subsequently used in many other movies.