Sometimes when I’ve been called to clear a blocked drain, customers will tell me about the puzzling noises their drain was making before it blocked. In fact their drain was already blocked and they just didn’t realise it. Not every drain will make this noise before the customer suddenly realises their waste water is spilling on the ground outside or their toilet won’t flush – but for the people whose drain is making noise, read on to understand exactly what’s going on.

If my drain is blocked, why doesn’t it just fill up with waste water quietly? Well, for most blockages this is what does happen. When a drain makes gurgling noises it’s sometimes an indication that the drain is blocked in more than one place. Other times, it’s simply because an unvented branch drain is blocked.

We need to get specific about some terminology now. Let’s say that a blocked drain is one that is truly plugged by a blockage that no water can get past (I saw a blockage like this just last week). Let’s call a partial blockage a choke. Most drains usually choke before they block.

Essentially a drain will make noise on account of either siphoning or venting. In either case, the noise is ultimately caused by air trying to escape from the drain or enter the drain. Think about the finger on the drinking straw – the liquid stays in the straw because nothing is available to replace the liquid as it tries to leave the straw but as soon as you take your finger off the end of the straw, air is able to move in and the liquid can move out. If you have a drain full of water and the water is able to seep past the blockage, it will only do so if air can enter the drain to replace it.

Ordinarily, a sewer vent is the place where air enters the drain to allow an ordinary flush of water move down the drain in a properly functioning drain. In most choked drains, this still happens and this is why most drains don’t make noise – but sometimes the vent is not where the water leaving the drain is pulling its air from. Sometimes the air will get pulled into the drain via your toilet or a floor waste gully or another fixture. This is called siphoning. Possibly less commonly, the opposite happens. This is when a toilet is flushed upstream of a blockage and the plug of water being flushed needs to displace air in front of it. If there is a plumbing fixture downstream of the toilet but still upstream of the blockage, the air will vent out through that fixture.

A clean, properly functioning drain will not make noise. If your drain is making noise it has problems.