This has happened twice in the past two months. When we run too many water appliances at once (dishwasher, laundry, shower, etc.) we will start to get gurgling in the pipes. Shutting off one of the appliances stops the gurgling. Lately when I have taken long showers, the gurgling starts again, even if nothing else is running. The gurgling happens to both toilets. Whichever toilet we flush right after a shower is the one that backs up, naturally. toilets do not overflow other times we flush - just when we've added a lot of water to the line immediately prior. We have suspected a clogged septic tank in the past when this happened. Septic guy cleaned out our tank but said that there was no clog. My husband has opened the tank a few times since then when the gurgling has persisted. No solids clogging, but the tank was full of liquid. We had a plumber come out for a different problem in January (six months ago) and mentioned the gurgling to him. He didn't think it was a septic tank issue then but I don't remember all what was said. Nothing backs up in the sinks/tubs. Any help?