According to the following thread http://www.avsforum.com/avs-vb/showthread.php?s=&threadid=144535 problems with hiss CAN be related to grounding issues.

Though the specific problem in this thread concerns balanced interconnects, the root problem and cause of the hiss here is lack of matching grounds, i.e. ground loops.

The thread also provides links to some interesting information on this topic.

Regards - Geno