Here's the scenario:
I have a 2nd floor bathroom sink drain. The sink has a typical 1 11/4 inch drain in it and has an overflow preventer port near the top of the sink. The drain line under the sink is a 1 - 1/4 inch chrome drain line P-trap that goes back into the wall. From there it makes a 90 degree turn down and goes down about 18-inches to just below the floor line. Then it makes another 90 degree turn to be virtually horizontal along the underside of the bathroom floor and goes about 8 feet and ties into the side of the 4-inch cast iron toilet drain under the toilet. Everything from the point inside the wall behind the sink and down and across under the floor to the toilet drain is approx 1 - 1/4 inch lead pipe with a horizontal curve in it to make the trip to the 4-inch cast iron toilet drain.
The sink is often very slow to drain. I have used a 50 foot 1/4-in snake to clean it out, which always helps -- at least for a while, But doing so is not easy due to all of the turns it has to make to get through the P-trap, down into the wall, and across to the toilet 4-inch cast iron drain. To make things easier, a few days ago, I cut into the horizontal piece of the P-trap and installed a coupling that I can easily disconnect. When I disconnect that coupling, the 15 fool 1/4-snake goes in easily, makes the 90 degree turn down easily, and makes the final 90-degree to horizontal and easily goes across to the end of the line where it drains into the 4-inch cast iron toilet drain line.
So, I was able to easily clean that line out that way. And, with the snake all the way in, I even slide the coupling and P-trap parts over the beginning (back) end of the snake and run the snake up through the sink drain. Then, I reconnected everything, ran hot water in the sink, and continued cleaning the drain out by pulling the 15-foot snake out and up through the drain with the hot water flowing to clean out the crud.
That worked (again), and the drain now drains quickly.
Meanwhile, here is my question:
Even though I did this, when the sink drains, I still hear a glub-glub sound. It's the same type of sound that one hears when pouring gas out of a gas can that does not have an open air vent in the back of the can. When pouring gas out of the gas can with the air vent open, there is no glub-glub sound -- meaning the gas goes out and air goes in behind it; therefore no glub-glub.
But, why wouldn't my sink draining operate the same as pouring gas out of a gas can with the air vent open -- with no glub-glub sound? When the sink is draining, the drain is open at the top and air can get in (just like having the gas can air vent open). So, why doesn't; that eliminate the glub-glub sound when the sink is draining into an open 4-inch cast iron toilet stack?
P.S. I have the ceiling completely open below the bathroom sink and tub, so I have an easy view and easy access to all of the drain line systems that are there. And, my plan is to re-do those drain lines in a better way that I know will completely resolve the problem. But, I am still curious why I am getting the glub-glub sound now when the sink drains into this open vessel (cast iron stack) with an open air inlet at the top (meaning the sink drain itself).