Very slight "Hiss"

The 'very slight hiss' you hear means you are still alive. I know of NO audio electronics that give ZERO hiss. Some may become really inaudible, but trust me, it is still there, just below your threshold of hearing.
The noise is just the stuff of electronics. Some are worse.. when you can hear it from your seated position THEN start to do something about it (which basically means throw money at it).

Elizabeth is correct. No electronics can achieve zero noise. What you guys are hearing is residual thermal hiss, which occurs in all electronics, regardless of quality or price. As long as hiss is inaudible from the listening position on the quietest music passages, there is no problem.

http://en.wikipedia.org/wiki/Thermal_noise

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Tweeters have a slight hissing sound at "Ear-to-Tweeter" distances when the amp is powered up. This is normal. If you other drivers are "humming" this is differant and is likely a ground problem.

Very common in various audio products.

Thanks to those who explained the nature of electronics and the fact that what I observed regarding "hiss" is normal.

Rpg