DIAGNOSIS That's the sound of the pump struggling, usually because it's trying to pull water through a blockage. The most common culprit is a clogged or incorrectly installed water filter.
FIX 1) Turn the machine off. 2) Remove the water filter from the reservoir. 3) Check the filter head—the blue plastic part that snaps into the reservoir. Make sure the blue rubber seal is present and not torn. 4) Soak the filter in warm water for 15 minutes, then rinse it thoroughly under the tap. This can clear sediment blocking the flow. Reinstall it, ensuring it clicks fully into place.
IF THAT DIDN'T WORK The internal check valve (a small one-way valve in the water line) might have a piece of scale stuck in it. You can try dislodging it by running several Hot XL water-only cycles in a row. The repeated pressure can sometimes flush the debris through.
HEADS UP If the noise changes to a high-pitched whine or screech, stop using the machine immediately. That indicates the pump motor is running dry, which can burn it out quickly.