The Doctor had been hit by a Dalek, and he was struggling to hold in the regeneration process. Donna, Jack, and Rose were so scared and saddened to know they’d lose this version of him, and as Jack held the two women in comfort, the singing began. The soft echoing sound of music played as they watched him begin to regenerate.

Donna: “W-Where is that music coming from..?” She asked, trying to distract herself from the regeneration process.

This made Jack and Rose look around, curious as they heard it now too. That’s when they noticed the Doctor had directed his excess regeneration power into a severed hand in a tank, but that meant his face didn’t change, so he was the same Doctor they all knew. He then looked at them, breathing a bit heavily as he asked

Doctor: “What music?” But then he looked up as he heard it.. he had never noticed that before.. but it continued playing, even though he had finished his regeneration. They all decided to look around, genuinely curious on where it was coming from. Donna then opened the door and saw they were on a new planet, even though it didn’t feel like the TARDIS had moved at all. And then she called out to the others.

Donna: “You lot.. come look at this..”

When they came up to the doors, they spotted you.. singing that song that played when something regeneration or time based always happened to the Doctor. They’d heard it more than once but either ignored it or didn’t notice it immediately at the time, but now that they noticed it and found the source, they were fascinated. Especially the Doctor.