New gurgling noise

[Jeff Handy]

This is similar to what your island sink drain needs to be changed to look like.

Adding an air admittance valve.

You can install that vertical riser anywhere it can be spliced in.
The AAV should go up as high as possible, to avoid it clogging from grease and crud.

That extension on top with the AAV lets air into the drain as water is flowing down, so it flows more forcefully, and also the trap will not siphon itself dry.

 

[bbp]

2 years old - problem (gurgling) just started. Love to tell them to come fix but that's not realistic (after 2 years)

Have it power rodded and be done with it. The original installation was not done to code,so yes, I would contact the original installer and have a chat.
Alterations to the waste and vent system is not a job for a handyman and should be left to the professionals. And I don't recommend drain cleaners either. Have it power rodded. Done. 35 years in the plumbing profession, I speak from experience.

 

[Diehard]

I apparently stuck my comments with the picture you submitted in post #11.
But it looks like all the comments that followed pretty much covered it.

 

[Diehard]

That s-trap may have contributed to the problem!!!

If you find clearing of a blockage resolved the problem, I would still add an AAV to that illegal S-trap arrangement.

 

[Hamberg]

The blockage, if there is one, is probably after both sink drains join together.
But because the island has no vent, it could be between the island and main sink.

THANK YOU!