What model turntable do you have? Does it have a phono preamp? Do you have it switched to “line” rather than “phono”?

You can ground the Port via the shield on one of the RCA sockets. I found I needed to do this when using Port in conjunction with a headphone amp.

It has a preamp and is set to line.  How do I ground to an RCA shield?  use thin wire and put in along side the outer ring?

Does the turntable offer a ground wire or ground lug? Built-in preamps don’t usually need a ground.

Yes, a thin wire under the RCA plug’s outer ring will complete the ground. You can quickly test this by touching the ring with the ground wire. If a ground connection between turntable and PORT is needed, the buzz will stop or reduce instantly.

Does PORT buzz without the turntable connected? Does the magnitude of the buzz follow PORT’s Volume control? What is PORT’s output connected to?

If you have additional SONOS players, disconnect the PORT’s output and play the turntable through another SONOS player. Is the buzz still present?

I use a spare RCA lead plugged into one of the unused RCA jacks on the Port. The other end is cut and stripped to expose the sheath. The core is tied off to prevent   accidental contact.

A RCA Y or splitter cable like this will work for that too and doesn’t take up a port if you need them all.

Cut one of the female ends off close to the female connector, remove an inch or more of the outside insulation.

Twist the shield wires into a single bundle.

On the center conductor pull the insulation back to expose as much as possible of the center conductor. Snip the conductor off and let the insulation extend back to cover the snipped end.

You can then solder or just twist in a longer ground lead and tape or better shrink-wrap the modified leg of the Y.

https://www.amazon.com/YCS-Basics-Splitter-Cable-Female/dp/B00EA8RCC2/ref=sr_1_18