When flushing my toilet the water comes all the way to the rim, sometimes it sits then drains slowly, sometimes I have to plunger it or sometimes it flushes ok but then the water goes way down to the siphon part of the toilet. I also have a shower in same bathroom that has started backing up and then slowly drains out. But the sink in same bathroom drains fine but all drains gurgle. I have a septic system 1500 gal tank emptied in 2016 had five people living here for 2 1/2 years only two now for a year. Looked in my septic tank and when toilet flushed or ran water I could barely hear a trickle coming from house. Stack, clog or main pipe clog?