I was in a sound proof corridor thing at the Ontario Science [del]Center[/del] Centre in Toronto years ago. Fun little walk.
Ears have an active feedback mechanism in the cochlea in order to amplify interesting sounds and deaden uninteresting ones (micro air movements, blood rushing, etc.). This system going out of whack can be one cause of tinnitus.
If you take away the background noise, for a while you will “hear” the uncancelled feedback in your ears. (I don’t know how long it takes for your ears to readjust.) In short, total silence sounds like low level noise.