The sloshing noise in your car may be you coolant being low. This will cause the noise because the cooling system if not full. The cooling system lines go from your radiator at the front of your car to the heater core that is mounted inside or under your dash. Any time that you have a leak in your cooling system then it can cause an overheating issue with your engine. This will also cause your heater to not work properly due to the low coolant.
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