Probably you have a plumbing setup where the drain from the toilet goes across to a main line. The tub drain also goes to this same pipe but enters below where the toilet line connects. At the bottom of the tub drain line is a trap (shaped like a you) which traps water in it and keeps sewer gases from coming up. A toilet has the trap built in to its base. When you flush your toilet, the water goes to the main line. When it goes down and passes where the line from the tub comes in it creates a slight vacuum which sucks at the water in the tub trap, thus creating the gurgling.
Its very simple. Your toilet line and your tub line are connected. You may have noticed in the past, or maybe not. If you get a plug in your sewer line, you could possibly get a backflow into your shower. The fact that your toilet and tub line are connected, is normal. Its the way plumbers set up the drain system in your house. You may have an older house, and that's why you hear the gurgling sound in the tub. The smell of sewer gas out of the tub tells me that you do have a backflow problem somewhere, and its not something that a jar of draino will take care of. You need to call a plumber.
You have a problem with the venting of your bathroom plumbing. It probably is improperly or not adequately vented. Have someone look at this because if the trap on your tub is emptying with the flush of the toilet, it could cause methane gas to enter the home from the sewer pipes. The water in you traps is what prevents that backup of sewage gasses into the house.
There are vents that go out the roof to vent the gases. Sounds like you need a vent or it is plugged. Anyway you should get a plumber to look at bit before you get a back up in the tub or smelly gas in the house. Depends on the house could be a easy job.
Also check your vent coming out of the roof. Debris can actually build up over time and clogg it up acting as a plug stopping your toilet from being able to drain properly.
Apparently there was something blocking the outside vent stack, causing suction to empty the trap in for the bathtub. Once this obstruction was removed the the suction or gurgling sound ceased. Thank all of the people that responded to this inquiry, because I was truly in a quandary regarding this matter.
