I have a six-year old Sony STR-DE835 which has served me well except for one thing. When I turn the volume down or when I select a source that is not being fed anything, i.e. selecting DVD while the DVD player is not on, there is an audible hissing sound, like the sound audiotapes used to make without Dolby NR.

So thinking that it was just my receiver, I went shopping last weekend and was surprised at what I found. I looked at the Harman Kardon AVR630, the Yamaha HTR5760, and a few others that I can't remember (they were all mid- to high level receivers) and in ALL of them, the same hissing occurred, even when the volume was turned down or muted or when a non-active source was selected.

Why is this? I was thinking that because I have a crappy Sony, this was an isolated problem but apparently not? What causing this hissing, even in expensive receivers?