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.
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.
