I am on a septic system and have three full baths. The toilet in the master bath bubbles when either of the other two showers are on. It does not bubble when the master shower, dishwasher, or washing machine are on and draining. I have Studor Mini-Vents under all sinks. In the master, I have two sinks (each with a studor mini-vent. When the bubbling happens, I take off the trap under the sink, air goes in and the bubbling stops. I have put Thrift in the shower drain to try and rid the main line of any clogs and there is no standing water in the showers or sinks. No other toilet is bubbling. In researching, I believe it might be the mini-vent but am open to suggestions.

This would be a bubble and not a gurgle,correct? Bubbles are blowback caused by a partial blockage. The solution would be to pull the toilet that bubbles and snake. The drain line. Let me know how you make out. Good luck, Tom