The Doctor and new companion Bill spent almost all of episode 3 enjoying a wintry romp in Regency London, but when they finally arrived home for a nice cup of tea (and coffee) there was a reminder of this season's big mystery – What's Inside The Vault?

The Doctor and Nardole have apparently been guarding the vault beneath St Luke's University for at least half a century, with the Doctor having sworn an oath to protect it (even though he’s been regularly slipping away for adventures since Bill turned up).

There’s clearly something pretty important inside – you only need to look at the giant metal doors to work that out – but until now we haven’t had much of a clue as to what it might be.  An artefact? A monster? Or something else entirely?

This week we got a pretty strong indication that, whatever it is, it’s very much alive.

As Nardole turned to leave after his latest visit, someone or something inside started banging on the door. Loud, powerful banging. Angry banging. And accompanying it was more banging. Rhythmic banging. Familiar banging. The sound of drums.

Maybe it’s just us, but those four repeated beats on the soundtrack seem reminiscent of the maddening drums implanted in the brain of the Master by the Time Lords, which we first learnt of when John Simm's incarnation attempted to take over the world as Harold Saxon back during the Tenth Doctor's tenure. They're slower, but otherwise very similar.

“Inside my head. I thought it would stop. But it never does. It never, ever stops. Inside my head. The drumming, Doctor, the constant drumming. It's everywhere. Listen. Listen. Listen. Here come the drums... here come... the drums...” – The Master

We've already conjectured that it might be the Master in some form inside that vault – after all, we know both Simm's version and Michelle Gomez's Missy are returning this series, and the markings on that door do look very Gallifreyan  – but this could be the biggest hint yet that whatever's behind it has bleached blond hair, possibly a goatee, and homicidal tendencies.

Of course, in the end, TV soundtracks drop drumbeats about as often as Doctor Who showrunner Steven Moffat drops red herrings, and we may well simply have fallen into his latest trap.

But for now, when we watch that scene back, and listen to those drums, there's only one person we imagine slamming his fists against the inside of that door with a maniacal grin on his face.

Heeeeeere's Johnny!

Doctor Who continues on BBC1 on Saturday