It's like a constant air blowing noise. It comes from all my Sony Bravia 5.1 speakers. I can only hear it when I stand only a feet or two away, more than that and I can't hear it at all. It doesn't increase with an increase of the volume, and occurs even when all other components, including the TV, are off.
Well I went to Futureshop and found my speakers on two displays. I put an ear against them to test and heard the same hiss. I tried other speakers and couldn't hear it (I only tried a few though). Seems it's confined to this particular model.