Customer is stating that the sewer line backed up when running the washer machine again, upon inspection the sewer line is flowing. I had the customer flush toilets an run a sink, waited about ten minutes with no issues and the sewer line did not back up. I recommend a video inspection of the sewer line to determine any issues but the customer declined. I believe there is a clog in the city main and at night when people are using there sewer it backs up into the customers cleanout. If the problem continues I highly recommend running a camera down the sewer line, running a snake wont solve the problem cause there is another underlining issue.