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.
