An engineer in the UK has developed a
silence machine which selectively eliminates predictable noises. The machine is basically a noise-cancelling device, equipped with a signal-processing computer programmed to find and cancel only those noises targeted, letting other sounds through.
A few years ago I heard about some production model car that had a noise-cancelling system installed - it somehow recorded the sound of the noise of the road / traffic, and played it back in reverse-phase through the car stereo. I think it was in a Honda. Don't know how well it worked.