septic - could you fill a 1500 gallon tank in 10 months?
Hmmmm....when we finished our basement, I was particular about noting the toilet venting to the plumber who was doing some other work not-related to the septic/bathroom. We had been experiencing this burping issue before this - so I noted it to him - he claimed to be all over it - he extended the pvc toilet vent higher in the wall so it was up in a void open to an unfinished storage room and put a new studer vent on top.
If in fact my main drain to the tank doesn't have quite enough slope, I'm not thinking it's totally clogged. Rather, solids are sitting in it, slowly building up. Water is flowing over them, and perhaps at times, enough water flows through to completely seal-off that main momentarily - causing a vaccum of sorts.
Something else worth noting - when I open the washout - I can see water in there mixed with what appears to be TP. And that water level sort of pulses or waves or bounces rythmically - like it comes up 1/16th of an inch and falls like something is on either end pushing a core of water one way or another...