I have back to back bathrooms with a toilet in each that comes down to the main drain in the basement. Just recently I could hear one and then sometimes the other toilet making a bubbling noise. The water level was down in both of the toilets. I flushed one of the toilets to see what would happen, and the water seemed to rise up rather high and go down very slowly. I watched for a while and eventually it bubbled up and the water level dropped again. Sometimes it takes only minutes for this to happen again after flushing, and sometimes it takes an hour. Right now they both flush and are draining quickly but they still lose water level and it acually splashes back up to the toilet seat from the bubbles. It loses water level very quickly. About 3 to 4 inches of water remains in the bowl. I had both toilets replaced about 2 or 3 years ago by a professional plumber. They are both water saving toilets Kohler brand name. He charged $1500.00. If it's the toilet how can I find out which one? Is it a partial blockage or venting blockage ?

Main sewer clogged