Newb question about air in pipes

[starfish_prime]

My house has a septic system. I have no idea how old it is or when it was last replaced/serviced/etc.. as we purchased this house about 3 years ago. We sometimes have an issue that I thought was a clogged pipe, but now I'm not 100% sure. The house has two bathrooms and one day the toilet didn't flush all the way. Obviously thinking the toilet was clogged I was like....F..... but I didn't have a plunger, and later the toilet ended up flushing (I wasn't around, and don't remember how long it resolved itself). Then a few weeks later it just happened that my wife and I flushed both toilets at about the same time and neither flushed. So now I'm thinking there might be a clog downstream. To confirm this I ran the shower as the drain for the shower is the lowest drain in the house. The thing is the shower wasn't backing up, then some air bubbles came out of both toilets and eventually they flushed. Fast forward about a month and this happened again. I flushed a toilet, it didn't flush, I went to test the other toilet, same thing, and so I started running water down the sinks and shower, bubbles come up from the toilets, and then they flushed. I should also note that each time this has happened I would flush the toilets again just to see if I could replicate it not flushing, but everything works fine.
So I'm curious, where should I be looking? Is this something with the air vent? Is there something I can do to troubleshoot? Any direction would be great.

Thank you

 

[GReynolds929]

You have a blockage downstream.

 

[starfish_prime]

Thank you for the reply. Can you explain why a blockage downstream would cause the air to come up like this through the toilet? And how running water in the sink and shower would help resolve this? I believe what you're saying, but just trying to understand it myself.
If it is downstream, I suppose I'll need to hire someone, or is there some easy-ish things to try to first resolve it yourself?

 

[Twowaxhack]

Water displaced air in a drain and when the drain is filling with water you will get bubbling in other fixtures.

No one knows your exact plumbing layout but you’re describing either a clog drain that’s opening itself as pressure is being applied by the pipe filling with water or there’s a problem at the inlet or outlet of your septic tank.

You need a drain cleaning professional to look into the cause.

 

[starfish_prime]

Thank you for this information. So it sounds like there isn't really enough space for the toilet water to go sometimes and then when you drain shower water down the drain it kind of moves the air back up to the toilet until enough pressure can bring all the rest of the water to drain. Obviously I have no idea how my system is set up either, just trying to get a better understanding and make it logically make sense to me. I'll be contacting someone soon.

 

[Twowaxhack]

As the drainage system fills up with water due to a clogged outflow pipe the air is forced out of the the outlet with the least resistance. In your case it was the toilet where the sue was forced out.