Sounds like a ground loop problem. I had a similar one. Audio Advisor sells a Mondial MAGIC cable TV hookup that deals quite well with this problem. Sells for about a $100.
Cable tv induced power hum
I have a Proceed AVP HT processor connecting EAD Powermaster 2000 amp, CD,MSB Link DAC, DVD, VCR, Phono, FM, Subs, & Cable, power connected thru Monster HTS 2000. If I switch Amp direct to wall outlet I have a huge hum. Thru Monster powerconditoner it quites down, but is still present. If I disconnect cable from VCR or TV & it goes away and am left with slight speaker hiss. How can I remove hum from my system? Do I ground cable, and if so how? I have no converter box. I have @home internet connection thru cable. Should I try some technique with sources other than Cable TV lead? Should I get another power conditioner or another type power conditioner to eliminate the hum? Thanks in advance for any assistance you can give. Larry Norton