Does your bathtub take a long time to drain, and does it make a lot of noise?
It is likely an issue with the plumbing vents. The sound is what happens when air and water are going different directions in the same pipe. Imagine what happens if you have a bottle of pop and you turn it upside down. The flow of the pop is interrupted by the incoming air.
Now imagine a little hole in the bottom of the bottle. The air would come in the hole, and the pop would leave the bottle smoothly.
That’s how plumbing vents work. (There are other reasons for plumbing vents, but let’s stick to this reason.)
But your bathtub glugs, and you want it to stop. Knowing the principle behind the problem is great and all, but to quote the prophet Paul Simon. “What are you going to do about it, that’s what I’d like to know.”
There are in fact a few paths to take. The first is to see if the bathtub is vented. Can you see a P-trap? A P-trap is different than an S-trap. A P-trap looks like the letter P and goes into a wall or into the side of a pipe that goes straight up and down. An S-trap goes straight into the floor. If you see an S-trap, the tub is most likely not vented. The solution there is to have a vent installed. A plumber might be able to connect the tub’s drain to an existing stack, but that is probably not an affordable option. Another choice is to install an Air Admittance Valve, or AAV. The issue with that choice is that Minnesota Plumbing Code has rules against using them. (Other states don’t restrict them, and they work just fine. It might be hard to find a plumber that is willing to break the rules.)
If the tub is properly installed, check to make sure the vent to and through the roof is clear. Tree seeds, leaves and small animals could enter the vent from the roof and get stuck. Or microbial growth could build up in the vent. Here’s a quick test. In the bathroom, close and seal the drains to the tub and sink. Seal the overflow vents as well. Then take a plunger and plunge the toilet. That will force air in and out of the vent, possibly clearing any obstructions. (Maybe not. Fill the tub and try again. Still glugs? Shoot.)
Okay, now it’s time to go onto the roof with a plumber’s snake, a sewer camera or both. This is not a sure-fire solution, because the lines have a lot of right angles. Cameras and snakes don’t take corners very well. Okay, at all.
So, you’ve done what you can. If the problem is still there, what’s next? If remodeling the bathroom is not on the horizon, you may have to accept the things you cannot change.