As I read your account I am reminded of an incident while repairing a tape deck on a military base. The repair was a success, but I suddenly noticed a periodic, raspy noise. I was completely flummoxed until, out of the corner of my eye, I noticed that the noise occurred every time a rotating radar antenna pointed in my direction.
I never assume a problem must be [ … ] or cannot be e … ], else I’ll likely be blindsided. I do, however, play the percentages. My first thought in your case is that there is an external cause.
Are you describing a noise that occurs while playing PORT through the receiver or when a receiver audio output is playing through PORT’s Line-In?
Does the noise level track the Volume control setting? Does the noise continue if you disconnect the audio input to PORT?
Now… that’s a great shout and it hadn’t occurred to me this being RF interference. I don’t know what it might be from and, of course, when I tried to do some tests this evening it wasn’t happening.
It certainly only occurs when the aux input used for the Port is selected, no issues when listening to vinyl for example. If there was strong enough RF to end up on the audio output I’d expect that to be affecting the 47 year old gear rather than the Port. I wouldn’t have thought it would make much difference which input was selected however it’s something to explore.
Historically, the SONOS ZP80, ZP90, CONNECT, and PORT can be easily victimized by ground loops. This is why I suggested disconnecting Line-In.
What is a “ground loop”? We assume that the case and the outside shield of all the signal cables are at the same potential which we assume is, zero, or “ground” for all boxes in a system. If, for whatever reason, one or more boxes are not at ground level, there will be an unexpected circulating current between boxes that can result in unwanted output. Large systems, such as broadcast and recording studios, can have massive ground loops and will hire engineers who make a good living picking through the equipment, eliminating the ground loops. After the ground loops are dealt with, there are very strict rules with respect to adding equipment because the addition might bring ground loops back.
Your observation that the noise does not occur for online music suggests there is an analog ground loop. Disconnecting the inputs will break the loop. A common creator of ground loops in home systems is the cable box. The cable box is part of a system that includes the neighborhood. If there is a grounding issue down the block, you will also own it if your cable feed is not properly grounded as it enters the house. There can also be lightning damage risks too. If you disconnect the cable feed and your noise stops, this is a very strong hint.
Miscellaneous noises can also be induced directly into the cables if the cable passes through a strong magnetic field. This is more typical of turntable signal wiring because the voltages are very low. If the wiring passes close to a power brick or power cable, the brick can induce noise into the phono circuit.
It can require some imagination to track down these little noises. Be careful with assumptions.
