Noisy ethernet cables

Also, to be clear, I'm less worried about noise getting INTO an Ethernet cable as much as it radiating to audio signal cables.

Except the Ethernet connector, metal shell, where the connector plugs into, is usually connected to the metal enclosure to improve the EMI.

That is entirely up to the equipment manufacturer.  I have a hard time believing a high end streamer maker would both ground for the sake of better EMI rejection AND leave themselves open to ground loops.  I certainly wouldn't design a mixed digital/analog device like that.

Many shielded ethernet cables ARE shielded on both ends, and that shield connects to case ground on both ends

Oh, so wrong. ALL shielded ethernet cables have shields at both ends.

The cable shield is end to end, with the RJ 45 connectors on both sides having a shield connection. That doesn’t mean the equipment maker is forced to use a shielded socket or to ground it. No shield connection on the socket, no ground connection there.

and that shield connects to case ground on both ends.

Only if the equipment designer makes it a choice to do so.

The shield on an Ethernet connection is not carried by the signal wires inside the jack, but on an outer foil like connector. It’s hard to see, but here:

As a streamer maker, you can either ignore that shield, or keep that shield separated from the rest of the system. You are not obligated to connect this shield at all, or to the equipment case.

Don’t confuse the shield on an Ethernet cable with the case and safety ground connection that is made to an AC outlet. They are two different things. There is no safety requirement that a signal cable be case grounded, and in fact lifting or isolating signal grounds whenever possible is good practice.

Only 1 end of an Ethernet connector needs to be shielded to work, and it’s nearly impossible to cause a ground loop via Ethernet cables. Ethernet signals are inherently isolated and balanced. There’s no galvanic path from them into the devices they connect to (barring PoE). I could see a streamer maker use a grounded plug which causes a loop, but that’s really bad design.

You probably want to stick with shielded Ethernet, regardless of the Cat #