[The Doctor turns towards Jamie's voice, his face still somewhat contorted from listening and trying not to listen. (It's not as easy as one may think.)]

That humming! I mean, we're familiar with the TARDIS' natural humming, but I've always been more in-tune with her...finer meanings. I can understand when she's upset, disapproving, cross, extremely cross, and so on and so forth.

But listen to this one. Really listen. [And he falls silent for all of thirty seconds because he seriously can't wait.] Did you hear that?