I have two bathrooms back to back. The toilets just started losing water level and making a loud bubbling noise and splashes up towards the toilet seat. I flushed the one toilet and it went down very slowly. Sometimes they lose water level in a few minutes. Sometimes it takes an hour before it bubbles and loses the water level. Usually there is 2" to3" of water in one toilet and 3" to 4" in the other toilet. Lately the water when you flush seems to go down like normal, but they still bubble and lose their water level. Both of the toilets were replaced about 2 or 3 years ago by a licensed plumber for $1500. Water saving style, Kohler brand. I would like to know if it's the toilet or toilets and how to chech each one separately? Is it possible that it's the venting. Is it possible it's a partial blockage in the sewer system? There is one other thing I noticed I could hear dripping in the main pipe but I did not see any more water leave the toilets and it also seems to bubble when my main sump pump goes on, or its just a coincidense because it doesn't happen all the time. I also noticed that when the water softner was cycling that is when the sump pump kicks on and the toilets did bubble a couple of times.

You have a main sewer clog