gurgling noise 1997 expedition dash
 

my wifes 1997 expedition is making a gurgling noise behind the dash at takeoff. i had the heater core replaced last year when it was leaking. this problem just began last week. i replaced the thermostat today. it did not fix it. no leaks found in the cooling system or floorboard. engine is 4.6L. heater and ac work fine. im stumped. any help appreciated. john