2003 Ford F150 water sound

when i take off or stop i hear water slushing around under the dash on the passengers side. my guess is at the heater core. but there is no water in floor board. but theres seems to be water coming from under the truch after i park. it has not been running hot. heat and ac still works good.

what could be the water slushing sound in the dash??

thanks twinky