It could be the fluid in the washer bottle moving around. Could also be air trapped in the cooling system passing through the heater core. Be sure the coolant reservoir is full and the radiator cap is in good condition.
The above is most likely...It could also be a clogged evaporator case drain, normally if it was bad enough to hear, the carpet when most likely be wet on front passenger side. An odor could also be present, due to the moisture causing mold/algea to grow on the evaporator.
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