Is it common for there to be a sloshing noise just behind the dash Dealership burped the radiator and now it's back?

Yes, a sloshing noise behind the dash can indicate that there is air trapped in the cooling system, particularly in the heater core. Even after the dealership burped the radiator, it's possible that some air pockets remained, which can cause this sound. It's advisable to have the cooling system properly bled again to ensure all air is removed, as trapped air can affect heating performance and lead to potential overheating issues.