for the last few weeks have heard sounds of water rushing through trap in basement. today, it was gurgling as well. removed trap plate, found the house side plug open--therefore the noise-- and a little sewage in the pit. this was during a washing machine cycle; gurgling seems to come at the end of most of the flow. Sewer line has to be cleared from roots twice a year; probably due in april. usually i can tell when I heard gurgling in the basement slop sink, but never in the house trap. i have to hope the top of the stack--through the roof--is not blocked, nor the vent just above the trap which goes through the foundation wall. so far no signs of usual backup into slop sink. We had roofing job last August, but if any debris was accidentally dumped at top of stack probably would have noticed problem by now. Any thoughts?