Love and a Piano
From Halopedia, the Halo wiki
"Love and a Piano" is a track not featured in any of the Halo Original Soundtracks. It runs for 1:49. Some Halo fans even call this song "Halo Romance" or "Halo Love Theme" because of the song's romantic feeling, which could be a reference to Cortana and Master Chief's "relationship".
The first section of this song can be heard very faintly as a backtrack on the song Ambient Wonder from the Halo: CE Original Soundtrack. Here the theme is played on a synth, with an approximately doubled speed and it is extremely faint and can almost only be heard in a silent room with good speakers, or with in-ear headphones.
The same section is remade into the opening of No More Dead Heroes heard during the opening of the campaign level Cortana in Halo 3. Here the timing of the note is slightly different, but the overall feeling is kept. A fitting use for the theme, as it is often used to represent Cortana's and John-117's "relationship", as it is used on the level where he is set to rescue her.