This has happened before. No one was using anything and there is only the two of use anyway. We are semiretired and I was sitting and I heard some bubbling in the toilet and I knew it was backed up again cause that is what happened last time. And when you plunge in one spot it causes things to happen on the other end of the house, even the in-law qtrs. We are built on a slab. Home is about 7 years old. There is, I guess, what you call a take out drain just in front of the house. When eveything was backed up I went outside with a monkey wrench and took the cover off and immediately there was like a vaccuum sound and then I could hear the water starting to drain. I went inside and my wife and I checked and the water was receding and the toilets were flushing again??

Today, I was relaxing in the bedroom and I heard that dreaded bubbling again and yup... we repeated it again. Wasn't quite as easy to recover even after removing the cover but it did after about 10 minutes, so guess, not bad. But doesn't make sense. Can't be anything clogged.

If you are on a city sewer it could have been a coincidence that just as you opened the clean out the water broke loose and drained. The same could have happened if you are on a septic system. The second time this happened and you took off the clean out it did not go down so fast, so that time it may have just seeped out of the line.

I really think you probably have a main line stopped up.

So readers understand... the city came out and absolutely no problems whatsover. They probed with a camera all the way to the main. Did a great job. Nothing coming up. Just stopping up for no reason. Also, have three vents in home.

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When eveything was backed up I went outside with a monkey wrench and took the cover off and immediately there was like a vaccuum sound and then I could hear the water starting to drain. I went inside and my wife and I checked and the water was receding and the toilets were flushing again??

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I have a different take on it then Letmetellu.
I don't think you have a blockage and the city backs me up.

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the city came out and absolutely no problems whatsover. They probed with a camera all the way to the main. Did a great job. Nothing coming up.

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I think you have a venting problem.
When you opened up the house cleanout, (take out drain?) and you heard a "vacuum sound" that was the line venting so it could drain.
What I think happened, (and this is just a guess) is that the city main backed up into your sewer line and was held there by vacuum until you opened the cleanout releasing the vacuum.
What disturbs me is that the vents in your house should have released the vacuum without you having to do it yourself. I would have the vents checked out.
If I'm correct and the city main's backing up in your sewer line a sewer check valve ,(see image) could be installed just ahead of the house clean out. Good luck, Tom

In my area the homes have an average of nine vents in a normal plumbing system so for the commodes to be stopped up because of a vacuum caused by a clogged vent there would have to be nine vents clogged for the system to be completely non functional.
Even if the vacuum was strong enough to hold water in the commodes the almost level pipe of the main line would gradually let water out enough to finally break the vacuum.

If and when you find out what the problem was I wish you would let us know.