Why Does My Sink Gurgle When the Washing Machine Drains?

It is a common sound in many kitchens: you start a cycle on the washing machine, and as it begins to pump out the waste water, your kitchen sink starts making a loud, rhythmic gurgling noise. Sometimes, you might even see soapy water or bubbles backing up into the sink bowl.

While a small amount of noise is normal in some plumbing configurations, a loud gurgle is usually a “warning light” for your drainage system. Here is what is actually happening behind your kitchen cupboards.

The Science of the Gurgle

In most modern homes, the kitchen sink and the washing machine share the same waste pipe. When the washing machine pumps out water, it does so at a high volume and high pressure. If the pipework is clear, the water and air move through smoothly.

However, if there is a partial restriction further down the line, the water fills the entire diameter of the pipe, pushing air ahead of it or pulling it behind it (creating a vacuum). This air has to escape somewhere, and the easiest path is often up through the water trapped in your sink’s “P-trap”—creating that distinct gurgling sound.

3 Common Causes of a Gurgling Sink

1. A Partial Blockage (The Most Likely Culprit)

Over time, kitchen waste pipes become lined with a thick layer of fat, oil, and grease (FOG), mixed with food particles. This narrows the pipe. While the sink might still drain okay on its own, it cannot handle the massive “slug” of water released by a washing machine, causing air to trap and gurgle.

2. Poor Pipe Venting

Plumbing systems need air to breathe. If your “vent stack” (the pipe that usually goes up through your roof) is blocked by a bird’s nest or debris, the system will pull air from your sink trap instead. This is often accompanied by a faint “sewer smell” in the kitchen.

3. Incorrect Pipe Installation

If the waste pipes were installed with an incorrect “fall” (slope) or if the washing machine waste hose is pushed too far into the standpipe, it can cause turbulence and air pockets in the drainage line.

How to Fix It

If the gurgling is a new development, you can try a few simple DIY steps:

• Clean the Sink Trap: Unscrew the U-bend under the sink (put a bucket down first!) and clear out any accumulated debris.
• Flush with Hot Water: Pouring a kettle of hot water down the drain can sometimes help soften grease build-up.

However, if the gurgling persists, the blockage is likely further down the stack where a standard plunger or store-bought sink unblocker won’t reach. In these cases, mechanical cleaning is required to strip the grease from the pipe walls and restore the full diameter of the drain.