manicdepression

I have my DD-5.1 hooked up to my Promedia 5.1's but when I hook a coax cable from my dvd player to the decoder I get a loud buzzing noise. When the decoder gets turned on the buzzing decreases but is still there. When I hook my Xbox up via optical cable I dont get this noise. I have the buzzing even when input select is on optical. Is this a known DD-5.1 issue? ANY help appreciated. TIA