A car making sloshing sounds as it turns a corner.
Illustration: Matthew Hollister

The most common cause is a clogged drain trapping water in the heating and cooling system. The water, formed by condensation, usually empties onto the ground.

But a clogged drain can cause it to back up, leading to the sloshing sound and even dampness on the passenger-side floor.

More on Car Maintenance

The fix is usually quick and easy but best handled by a mechanic.

Most vehicles have several drain holes, including in the doors, trunk, and sunroof. When they become blocked, water can collect and cause damage, including corrosion. If the drains are clear, the cause could be the engine cooling system, which should be checked by a professional. 

For related car-maintenance information, watch our video on car maintenance myths.

Editor's Note: This article also appeared in the October 2019 issue of Consumer Reports magazine.