Have you ever banged away at a piano and then suddenly stopped? If you listen to the sound die away you can hear a whine or a hum. It lasts for a second or two.
The sound in the room is a bit like that except it's the same sound all the time and it doesn't change. It reminds me of when we were on the steamer at Portsmouth--only a thousand times quieter. Or like hearing the sea in a shell when you put it to your ear.
Or that strange, roaring sound I heard once when I went into a tunnel.
I think I'm in a dream. No one seems to notice me. I carefully stand up to look around the room.