The sloshing would be an air bubble in the main part of the system. Each time it is completely cold and parked on level ground, take the radiator cap off and top up the radiator. With 50:50 of course.
Taking coolant out isn't a good idea here.
The coolant is always full up to the neck though, it's never low. I most fool with the "quart" that is in the overflow for OCD purposes. Car has heat and everything. Might have gotten better but if driving around with the heat on is the way ?
Should I get (another) Lisle funnel?