Started noticing a gurgling sound after shutting the vehicle no codes

I can hear surging in top radiator hose and see fluid rise in expansion tank but there are no code thrown. Temperature gauge usually stays at 210 F most of the time, except recently after driving around seventy five mph on a two hundred and forty mile road trip the temperature creeps about two tick marks up on the temperature gauge. This does not happen until about twenty five miles towards the end of the trip and has happened the last two trips made. I am getting ready to replace the thermostat and lower hose. I have looked at the coolant level and is full and watched to see if any air bubbles in radiator and see no bubbles. Why is the temperature very slowly creeping up? The surge noise is very distinct and you can physically feel in top hose of radiator after turning off the vehicle. The are no visible leaks underneath the car. Any help would be greatly appreciated.