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The last few days the toilets in the house (2 toilets) are making a gurgling noise. When I go and look, I can see the water is standing very low. When I then flush the toilet, the water either keeps rising (not going down the pipes) or flushes normally. And then there's about a 50/50 chance of it to start gurgle again.

We've tried unclogging it with a plunger, but it does not help. So I'm suspecting it has something to do with the septic tank.

What should I try to fix this?

EDIT: I should also note that one toilet is on a floor higher then the other.

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Have the septic system serviced, it may be full or nearly so. –  Tester101 Jan 20 '12 at 19:47
Your hearing the gurgling sound from toilet or sink is there any bubbles comming up when flushed? We're do you live cold weather? –  Mark Jan 21 '12 at 0:02
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4 Answers

up vote 7 down vote accepted

I've been a victim of this before. This is bad news.

In my (rare) case, the roof of my apartment building drained to the middle of the building and down the sewer pipes. During phenomenal downpours (twice in five years), the flow of water coming down that pipe was faster than the flow of water leaving the building. The water backed up to the lowest point where it had an exit which was my primary bathroom. It did so with such intensity that water was violently sloshing out of the toilet bowl, the stopper on the sink was forcefully ejected, and the bathtub was backing up at the same time. The only good news in all this mess is that it was fresh water.

Clearly, this is not quite your case. However, it is an indication of the same similar fact: you have a flow going in the wrong direction in your sewer line. Before the astonishingly violent discharge and water flowing everywhere, the toilet would gurgle.

Positive water pressure in sewer line

Since it's a septic tank for you, you need that serviced now.

The reason the flushed water level is lower is probably that the siphon link between the bowl and the sewer line isn't being broken.

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It needed service indeed. We've opened the tank and saw that the drain pipe was completely clogged by undigested toilet paper. We've cleared the top of for now. But will likely call some service to empty it ;) –  Simon Verbeke Jan 21 '12 at 11:29
Good action shot with that pic. I would not be very happy if that was my sink! –  Steven Jan 22 '12 at 22:37
For anybody ever reading this in the future: be careful about opening your own septic tank. Sewer gases can be fatal. –  Jeff Ferland Oct 1 '12 at 4:35
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It sounds like you have a blockage in your vent. Check if any birds have been nesting on your roof. You could also try snaking out the vent, but this really depends on where exactly the blockage is.

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The gurgling noise and the low water level in the toilets is because the pressure in the sewer line is lower than outside. The water is sucked (or more physically correct, pushed by air pressure) down the toilet.

When you flush, you add volume into the sewer, which is sometimes enough to even out the pressure difference, sometimes not.

Normally there is a release valve/pipe of some kind somewhere, where the pressure can even out between the sewer system and the outside air. This doesn't work properly in your sewer, making it a sealed system where pressure/vacuum isn't released.

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I had the same problem but it turns out the top to my stack had a hole allowing rain and debris to fall down into it but that wasn't the issue. My two younger children decided it was a good idea to throw gravel down it, causing a blockage which I had to remove (messing up a pair of gloves in the process) and as soon as it was cleared it began to flow properly.

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