The TARDIS took off with a loud noise, something like a siren, but not really anything at all like a siren at the same time.
"Why does it make that noise?" asked Rose Tyler.
"Why?" asked the Doctor. "Does it bother you?"
"No, but," she said, "it is very loud."
"Oh," he said. "Sorry." He pressed a button. The TARDIS landed at their destination—London, England, Earth, the Milky Way, current day. However, this time, there was no not-quite-yet-very-siren-like noise accompanying it.
"You can just turn it off?" asked Rose incredulously. "Why does it make that noise, then?"
"I'm too lazy to turn it off," said the Doctor, shrugging.
Rose looked at him sideways. "Why's it on to begin with?"
He grinned. "Would you like to find out?"
"Of course I would," she said.
"Well, then, we'll go to your street, in front of your apartment, four hours ago," said the Doctor cheerfully. He pressed the button again and began running around the console of the TARDIS, reaching his arms out as far as they would go and sometimes even using one of his feet to push buttons, levers, and switches. Rose had to laugh, as she usually did, at the ridiculous display of turning the ship on. Then, as usual, she fell over as the TARDIS shuddered into the Vortex.
A minute later, with that odd, indescribable noise, they appeared on the street right in front of Rose's apartment building. The Doctor hit a button, and the television screen played feedback from a hidden camera in Jackie Tyler's room. Jackie sat up as the TARDIS sound became louder, and then buried her head under the pillow. Even through the muffled quality of her voice and the noise, she could be heard clearly. "Can't he make that thing any quieter?!"
In the TARDIS, Rose and the Doctor doubled over, laughing. "And that," said the Doctor in between laughing, "is why the TARDIS makes that sound."