Singing
Harry, Ron and Hermione trudged the last few tiring stairs up to Gryffindor Tower. It had been such an odd, poignant Welcoming Feast without Dumbledore there to greet them for the first (or last) time. They hadn't even meant to come back to Hogwarts; but JKR had promised Harry that her series would be about his "Seven Years at Hogwarts," so they had to come back. (Despite what some people believed.)
They made their way to the portrait hole, expecting to see the Fat Lady. But – to their shock and surprise – there was a portrait of Dumbledore, covering the entrance!
"Password?" the old wizard winked, his eyes twinkling.
Harry was too stunned to speak, so Hermione nervously piped up, "It ... it ain't over till the Fat Lady sings?"
Dumbledore said, excitedly, "Aha!" and turned and ran away from them into the distance in his picture, getting smaller and smaller. There was a hut on the hill in the distance, and the old Headmaster ran up to it, opened the door, entered and slammed it shut.
The trio looked at one another, puzzled. The rest of Gryffindor House was starting to back up behind them, grumbling.
Suddenly, a female voice came from a distance in the painting, seeming to emanate from the hut. It sang, "Ah, Sweet mystery of life, at last I've found you..." And the portrait door opened with a click.
"That randy old bugger!" muttered Ron.
"This'll slow down midmorning breaks considerably," commented Harry, as he entered the common room.