Jdyer may be on to something. I ran into a similar problem and found that I did not have a good ground. Some receivers will have a separate ground terminal that can be attached to a ground source. For a simple test, run a wire to a good known ground or to the chasis of another known grounded (uses a 3 prong plug) device to the ground terminal on the receiver or to a case screw thats bright metal.
In my case, the ground wire in the AC wall plug was never connected when they built the house. By powering off the circuit and fixing the plug (only try this if you know what your doing otherwise hire an electrican to check your ground circut.) the hiss totally disappeard.
Hope this helps.
