The new jeep 3.6 is a TERRIBLE engine. Good luck. Jeeps nowadays are in no way the jeeps the past. And a bad thermostat isn't going to cause a leak, unless it's stuck closed causing a leak the you will be easily be able to find, also that fact that a permanently closed thermostat will cause it to overheat. But the housing and or the gasket could be no good. The sound you are hearing is coolant and air sloshing around in your heater core.