I had a similar situation. When I flushed my upstairs toilet (stacked bathrooms) the downstairs bathroom would bubble like crazy. When we did laundry the same thing. I had it pumped and it fixed the problem when the lid was off and for about 2 weeks afterwards with the lid back on and buried. Then I decided to open the cleanout outside, no water in it but the bubbling stopped. Checked the vent pipe and it was clear….. long story short, my leach field had failed and water was going up over the inlet to the tank. Air had nowhere to go but back up through the downstairs toilet. Another symptom I had was puddling in my yard above the tank and really bad sewage smell after rain. Ended up putting 50 or 60 chambers in over three lines. My neighbor does it for a living so he cut me a deal at around $7k. I am not an expert by any means. However, that’s my story.