Sketch out a diagram showing all audio components and all of their respective cable connections and power connections.
Include everything that has audio. Also include power strips, surge protectors, etc..
Any loops? I.e., A connects in some way to B which connects to C which connects to D which then connects to A.
Next, as methodically as you can, try removing each respective device from the loop. Change/remove only one thing at a time.
Determine if the interference ends or changes in some manner.
Objective being to narrow down to one device or connection path.