When exposed to noise for prolonged periods of time, sometime in the middle noise becomes silence. You don't feel there is noise, unless some turns it off or increases/decreases a little.

Its called conditioning the mind.

Its a bit like the right pane in eclipse, In years I never remember using it even once, yet its absence will irritate me.

Ever get that feeling sometimes when you are surrounded by people in a noisy place and yet after sometime sort of don't even feel that is disturbance.