I bought an Onkyo TX-8050 stereo amp a few months ago, then it died and was replaced, and having now had the replacement unit for a few weeks I've just noticed something strange that I'm a bit concerned about.

There isn't any noticeable problem when the unit is playing at normal volume, but when it's at very low volume or on mute I notice a faint static hissing from the right channel, and a similar but more scratchy sounding noise from the left channel. It is faint enough that I have to sit close to the speakers to hear it clearly, but I still occasionally notice it a bit during normal listening at low volume. The problem occurs with headphones too, which obviously rules out a fault with the speakers. I don't think it's always been like that because I'm pretty sure I would have noticed it before now if it had.

Is this normal, or am I the unfortunate owner of a second faulty unit?