Diagnosis
This is amplifier noise. It is often audible in a very quiet room. It is not a sign of a defect in most cases.
How to Fix It
First, enable the "Line-In Auto Play" setting. Open the Sonos app. Go to Settings > System > Your Era 100 > Line-In. Toggle "Auto Play" to OFF. This prevents the internal amp from constantly monitoring the line-in port, which can cause noise.
If That Didn't Work
Check for electrical interference. Plug the speaker's power supply directly into a wall outlet, not a power strip or surge protector shared with other devices. Move the speaker away from routers, cordless phone bases, or large transformers.
Heads Up
Some level of inherent electronic noise is normal with Class-D amplifiers at close range in silent conditions. If the noise is loud, changes with volume, or is a buzz/hum, that indicates a different problem.