I am having this issue too. I upgraded my AVM30 to the AVM30-HD back in mid May and when I got it back I was getting this hiss (along with some other wierd issues). This was with the earlier firmware 1.31
Brought the unit back, they replaced the DSP board, returned the unit to me with 1.33 installed and the problem was still their (although not as severe). My hissing/buzzing happens the loudest out of the front left speaker, but it is present on all channels.
After many tests to isolate the issue (originally suggested it was my projector) we concluded it was the AVM as I was able to duplicate the problem with no output connected at all and only one HDMI cable from one player to the AVM, and tested it with 4 different players. Drove down to Anthem (about 1hour's drive for me), brought a couple of my players and was able to duplicate the problem for them. We even tested with their DVD player and was able to duplicate the issue.
I'm told now it might be a software issue and patiently waiting. It is really annoying as it happens when there is a break in the HDMI signal ... whether changing HDMI sources, the chapter breaks at the beginning of a DVD or HD DVD, some menu changes on a DVD.
They have my unit to test with now so hopefully they will have a resolution soon. My movies are stacking up in my backlog as my theater has been down for just over 7 weeks now
Anthem support has been great in trying to resolve this issue but I want my theater up and running again.