Two words: white noise. My favourite is the linux SoX (Sound eXchange). sudo apt-get install sox with this command (which varies the sound, rain-like): play

Two words: white noise. My favourite is the linux SoX (Sound eXchange). sudo apt-get install sox with this command (which varies the sound, rain-like): play -t sl -r48000 -c2 - synth -1 pinknoise tremolo .1 40 < /dev/zero