I'm having the same problem with my 970/7075 combo although I have to be pretty close to my speakers to hear the hiss.

1. It sounds a lot like the hiss from the old cassette tapes and only occurs on the tweeters.
2. It occurs no matter what input is active even on input with nothing attached.
3. It is somewhat linear with respect to the volume knob position (As I go from min to max on the volume knob it starts out constant than at about half way up it starts to get louder until it's pretty obnoxious).

I'm guessing the noise is being generated internally to the 970.

It's either a poor job of decoupling the DC power supply which I can't do anything about.
OR
It's noise being injected through the back panel ports. I'm thinking about trying some RCA Shorting Caps on the unsed inputs. Other forums point to unused RCA-type digital audio inputs as prime noise generators as they are high bandwidth inputs.

I've ordered a handful of these to try out:

RCA Port Cover/Shorting Cap

I'll post again after I try them out.