A knell is a ringing sound, particularly from a bell tolled to announce a death or the end of something. Which is kind of depressing.
From the Old English cnyll, meaning “sound made by a bell when struck or rung slowly,” comes our modern day knell. It certainly describes the slow, ominous sound of funeral bells, but isn't always used so literally: We often say that a final blow or action that will bring an end to something sounds or signals the death knell. And if you hear a bell knell in your dreams, look out — superstition says that's not a good sign.