Who is knocking up The Doctor?

Digital Spy
"It is returning, it is returning through the dark ... he will knock four times." The psychic lady on the 200 bus said these darkly prophetic words to The Doctor during the conclusion of 'Planet Of The Dead', whipping up the tension ahead of David Tennant's impending departure. But who exactly wants to get in touch with The Doctor? We look at some of the possible characters who might be flexing their knuckles (or tentacles) in advance of the knocking...
Read the rest here.