She starts making the whirring, grinding sound of the TARDIS again with a knowing smile. It isn't just vocal chords making the sound, but her spirit, if you could call it that.

"Though I do think this may be the first time you've considered me a person." All-in-all Sarah is one of the Doctor's companions she's more fond of, and so, like the Doctor was prone to do, she wanted to coax Sarah into figuring it out for herself.