As envisioned by director Anna Adams, this is one large piece of carpentry to make the sound that it does -- there's a warning posted in the theater's lobby that's usually reserved for gunshots and nudity. It's as if this particular door is weathering some kind of natural disaster for the 50 minutes we hear it.
But mostly, it's just a cheap device that allows Earnshaw's characters to talk around what's really on their mind. Earnshaw seems to be aiming for Beckettian existentialism, but the exercise simply comes off as rather juvenile.
We hear the first thwack before a word is uttered, and then Boyd launches into a rant: "Drives you round the bloody bend, doesn't it. Round the bloody bend...the bloody door, always bloody banging. Listen...Silence. Then just as you relax." That's when the door slams again in predictable fashion, followed by Ryan telling Boyd to go shut the door.
This exchange is repeated countless times, sometimes with the roles reversed, to the point of irritation. Perhaps, Earnshaw is attempting to create the gravitas that Beckett builds throughout Waiting for Godot, so that by the final moment when Vladimir tells Estragon, "yes, let's go," nobody expects them to move, but Earnshaw never makes these moments feel more than mere theatricality.
Fortunately, there's a story between Ryan and Boyd (a soldier drama of sorts), and Cobley and Westgate relish the dialogue when they get to the heart of it midway through the play. However, their best efforts can't elevate it beyond trifling melodrama. In these sections, the door bangs less, highlighting just how forced a device it is.