After driving for a while and you shut it off you can hear the water like boiling

But it can sit in my yard running for hours and not run hot. Changed thermostat and can't see any leaks. No water in the oil. After driving for a while and you shut it off you can hear the water like boiling in the overflow reservoir (at least that's where it sounds like it is). One of the fans runs all the time but one comes on and goes off.