Is anything connected to Line-In?

Nothing is connected.  It is plugged into the wall for power and it is hooked up to wifi.  My system is over 2 years old and this just started happening a month or so ago.

We cannot rule out a hardware issue. Does the noise level track the Volume control? Does the noise stop after the music stops? As I recall the FIVE will shut down its amplifier two minutes after a track stops and  playing.

The noise level does not track the volume control.  It does it if the music is on or off.  

Very likely this is a hardware issue. You’ll need to work with SONOS phone support. Submit a diagnostic before you start a music playing session (the unit has been silent for 10 minutes or more) and again while music is playing.