When I run two loads of laundry back to back, I am hearing gurgling in the downstairs shower of a split foyer home with bathroom on ground level slab. Sometimes the water rises up in the shower and then recedes immediately. The toilet downstairs is turned off with the level of water in the bowl lowered, as it sometimes rises in the toilet and flushes on its own. I have a septic tank that was last emptied two summers ago. Over the years I have on rare occasions smelled a foul odor from the shower stall that never was used. I would pour fresh water into the drain and it remedied the problem for a long, long while. The shower has been used sporadically over the last several years and I have not noticed a smell. I am not looking to have to hook up to the sewer unnecessarily. I need sound advice to be able to converse with the plumber and not get taken. Thanks