strange,...i have one of these
(http://www.radioshack.com/search/index.jsp?kwCatId=&kw=loop%20isolator&origkw=loop%20isolator&sr=1)

but problem was still there, however, problem was gonne when i disconected all unused input cables from the back,... i guess some other device, TV or IRD was making a bad ground connection to z5500

tnx guys for help!!