Sometimes it's the AC adapter causing the noise.
I had a similar problem with my SNES until I switched Power Supplies.
That Cheap 3-in-1 AC adapter that supports NES, SNES and Genesis you see on eBay is an example of a bad adapter as it causes alot of noise.
Another issue is the AV cables you could be using (analog formats only).
The RF switch is a source of alot of noise and interference, switching to composite video reduce this to a degree, same with a RGB cable.
The Quality of cable also effects this as well, such as a shielded cable outperforms a cheap thin cable.