When Ian & Barbara are searching for the Sensorites, after they go through a door, the whole wall jumps as if a stagehand bumped into it.
As a Sensorite takes away the TARDIS lock, a cough can be heard in the background.
When Barbara is talking about London to the crew of the spaceship, she describes Big Ben as a clock. Big Ben is the largest bell in the clock tower of the Houses of Parliament.