I've had this problem before and paid a plumber to clean it out. We've got a laundry room cleanout on the lower floor. It seems that the upstairs kitchen sink drain is also connected. We used to have problems with the kitchen sink filling up, but not any more. However, what I see now is that water shot up through a sink in the laundry room and is filling up the washing machine. I'm removing that water by hand and dumping it. It smells like dishwashing detergent. I tried cleaning out the cleanout with a 25 ft plumber's snake, but that's not doing anything. When I remove the plug it will often shoot water up and slightly flood the floor, but it's slightly inclined and moves away. I don't think it's creating a problem with our main sewer line. If it was the waste from the toilet would be coming out of the cleanout. Like I said, it smells like dishwashing detergent. If there was human waste in there I'm sure I could tell. I'm not quite sure how it all started recently other than I had problems with the kitchen sink flowing slowly, so I used a plunger to try to clear it out. When I first opened the cleanout plug, I saw some crusty stuff come out as well as some brown sludge. There's also a lot of sand that shot out of the laundry room sink. I'll probably get a plumber, but that might take some time to get an appointment. Once it's fixed, should I used one of those enzyme build-up removers? I had a little bit left and tried dumping it in the cleanout, but I'm not sure how fast that would act.