Noise with no signal?

I seem to have a bit of constant noise(hum)
independent of volume level. The suggestion of unplugging the TV cable seemed to greatly reduce the level of noise. I am unsure of the "ground loop isolator" solution though.
All models I have seen are rca type. The 755 manual doesn't specify where the isolator is installed. I have been looking for one to put between the TV coax input and wall cable output. Is this correct? Or is the ground loop isolator installed between the amp input and pre amp out?
The noise is not too noticeable with normal volume from system but is quite noticeable during quiet passages.
Thanks,
Richard

Pick up two 300ohm to 75ohm video transformers at Radio Shack. Connect them back to back by connecting (by soldering, preferably) the 300ohm (separate wire) leads together so that you can then connect it (using one additional piece of coax) between your wall plate and the coax input of your TV/processor/etc. It should cost less than $10.