_Somewhere_ underneath that is something with feet. Feet that make pitter-pattering sounds. Feet that almost certainly belong to a mouse*.
Or, at least, rational, sensible Andy thinks they almost certainly belong to a mouse.
To the highly-sensitised ears of movie-watching Andy, they are _clearly_ the movement sounds of a Facehugger.
*Current nickname: Tarquin. Julie says I shouldn't name the creatures that occasionally visit the flat. But other people name cars and computers, and I don't do that, so naming living things seems reasonable to me.
Original post on Dreamwidth - there are comments there.