When noticing this atmosphere I am in the situation of a man who is working in his room, reading, writing, speaking, etc., and who suddenly concentrates his attention on some soft uniform noise, such as one can almost always hear, particularly in a town (the dim noise resulting from all the various noises of the street, the sounds of wind, rain, workshops, etc.). We could imagine that this man might think that a particular noise was a common element of all the experiences he had in this room. We should then draw his attention to the fact that most of the time he hadn't noticed any noise going on outside, and secondly, that the noise he could hear wasn't always the same (there was sometimes wind, sometimes not, etc.)