MISS PATTY: It’s your wedding day. Feel each other. Use the thumping of your heart as a metronome.
A metronome is a device which produces an audible beat at regular intervals. Musicians use it to practice playing to a particular tempo.
This is a music-heavy episode, and the connection between the heart and metronome shows how closely aligned music and emotion is. One of the themes of the episode is how Lorelai must learn to control her feelings to maintain her relationship with Rory, just as the metronome controls the beat of the music.
The Annotated Gilmore Girls 