Four Knocks

"Four Knocks" was a musical piece composed by Murray Gold.

It was used in The End of Time Part 2, as the Tenth Doctor saves Wilfred Mott from the Immortality Gate's control rooms, which are about to flood with radiation. The piece itself captures the Doctor's realisation that saving Wilfred would kill him and the emotional breakdown that follows.

The piece was used as a leitmotif for death and parting - particularly regeneration - and can be heard in The Time of the Doctor, as Clara reunites with the dying Eleventh Doctor in Trenzalore.

Outside Doctor Who
It was used in the prologue of TeamFourStar's Dragon Ball Z: Abridged Special - The History of Trunks.