It will need some investigation work to find out where the interference is getting on the system.
From what you have said I would start looking for a ground loop initially, so a good start on commercial is things like the cameras mounted on metal surfaces.
Ground loops are the type of thing that can effect multiple channels when there could only be one channel at fault so disconnect any with a fault until all is clear.
Testing for AC in the mV range on the cable is another way to locate. Test and reconnect until you narrow it down.