Toilet gurgling...but not all the time

[Samui]

Hello all. I recently started hearing the toilets sometimes gurgling when the master bath is draining or the washing machine is emptying(they share a wall between them) and upon inspection the water is gone from the toilet bowls in the toilets that also share that wall. From what I have read it sounds like a drain/waste vent might be clogged. Here is what I have done so far. Single story house with a septic system btw.
I went on roof and used a wifi endoscope and went all the way down until the pipe made an angle the scope couldn't follow approximately 15ft. No blockages noted and I could see several takeoffs but couldn't make that sharp of an angle to go into each one but no blockage noted where they joined the main line. I took a hand auger/snake and very gently went down to almost 25ft (the auger is only 25 feet long), no resistance, no blockages noted and the auger tip was clean. In the laundry room there is what appears to be an inspection plug that I unscrewed and put the scope in and traveled upward toward the roof as far as I could approximately 6-8ft and no blockages seen.
In addition the toilets don't gurgle all the time. Sometimes a couple of loads of laundry and a couple of showers a day and nothing....then one day just take a shower and the toilets gurgle and empty. So...I have run out of ideas and hope someone can point me in the right direction.
Thanks,
Jim

 

[Jadnashua]

Next time, try to see if it correlates with a heavy rainstorm. What you may be seeing is that your leach field is saturated, and there's no place for the waste to flow to easily...IOW, the drain line is essentially full.

 

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[Samui]

Thanks for the reply. It doesn't seem to correlate that well. It seems random or at least I haven't figured out the pattern yet