Get your drain scoped. If you have clay between your house and the sewer main root intrusion could be the culprit. Watch out for sketchy plumbers. Had our drain scoped by pulling the toilet. They reseated the toilet and it leaked. Wouldn't back up their work and wanted another $90 to reseat the toilet. Told the plumber he could leave and then did it myself. Snaked the toilet, the washer drain and then ran a garden hose through all the drains and the problem is now gone. The only good thing they did was let me know I had abs between the house and the main. So - get a REPUTABLE plumber.