Best episode of the season so far.
I really liked the subversion of the usual trope, as there were various noises throughout the time travel pod. Metal contracting and expanding, knock on the door, etc. Each time, The Doctor gives a rational, scientific explanation for the noise. Which, this time, apparently is right.
As a TV audience, we "know" the mundane explanation for the strange noise is never right. That expectation was played with quite well, I thought.