We recently moved into a 40 year old house with a kitchen fully renovated in around 2007. After a few days, we noticed that at the sink of the kitchen island, water drains okay for the first several seconds, then the water level rises. After that, it starts gurgling for an hour before slowly drained completely. It is an island sink, there is a bow vent or a loop vent below the sink (please see the attached photo). What I first did is replacing the garbage disposal, only because it was leaking. Then, I put a 25 feet snake completely through the cleanout at the vent side. It helped a little, but we still have the same problem. After searching through the internet, perhaps what I have to do next is a putting a garden hose from the roof vent. However, I figured that there is no vent pipe coming out of the roof. There are some pipes running horizontally in the attic, but not sure which one is the right one. My another concern is that even if I run a garden hose through the vent from the attic, probably it will go down vertically to the main vent, instead of turning horizontally to the island sink. Another option might be cutting out the loop vent and installing an air admittance valve (AAV) above the P trap. But, not sure what to do with the other side. Putting another AAV or an end cap? Any comment will be greatly appreciated.