Neil Robinson's poem starts with something very ordinary, the sound of an extraction fan, and then stops to argue with its own word choice. The speaker says "hummed" isn't right, because humming suggests someone cheerful and carefree, a person content enough to make a little tune. The fan has none of that. It isn't happy or relaxed, and there's no bliss in it.
Yet the final line returns to the same word: "still, the fan hummed." That small reversal is what gives the poem its feeling. The fan keeps going whether or not it means anything, and the repetition suggests a dull, steady persistence. The speaker seems a little weary, perhaps noticing how a sound can seem pleasant while being only mechanical. The poem could also hint at people who carry on and keep up a cheerful surface without feeling it, though that reading is an inference. The mood is quiet, flat and faintly melancholy, with a touch of dry humour in the speaker's fussiness over one word.