I've heard that since my first stereo. Take almost any amplifier and with no input crank up the volume and you usually can hear that noise. Some say it's from The Big Bang. Most of the time it's just the (electrical) noise around us and in the amplifier.
http://www.xprt.net/~rcrowley/tvcaudio/humnoise.htm and thousands of web articles are available.
There's a strange noise coming from my speakers most similar to the sound a bat makes. I'm curious to know what could cause this. I've included a small sound clip. Thanks for any feedback guys!