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The AC system, in a nutshell, consists of 4 main components. The COMPRESSOR draws in low pressure refrigerant gas, and boosts the pressure. The high-pressure refrigerant gas then goes in the CONDENSOR, a radiator-like component that removes heat from it and causes a change from a gas to a liquid. From there, the high-pressure liquid refrigerant is fed through a restriction of some type. The liquid refrigerant , at ambient temperature , wants to flash into a gaseous state, which, like boiling water to make steam, requires heat. By forcing the liquid through a restriction, or orifice/expansion valve, the rate at which it reverts to a gas can be controlled, and more importantly in your case, where the heat required to do so comes from can be controlled. There is another radiator-type component under the dash called the EVAPORATOR CORE, which, as its name implies, is where the high-pressure liquid refrigerant is allowed to evaporate back into a low-pressure gas, and by doing so, the heat is removed from what it touches, this Evap core. We blow air across the evaporator core and-bingo-cold air. Now, that evaporator core is the thing that condenses water on it, much like a glass of ice water sitting on a table will.As long as the temperature of the Evaporator core itself stays above freezing, the water condenses and runs off. However, if the Evap core temperature drops to BELOW freezing then that condensate can freeze on the evap core, blocking the airflow across it.There are switches and sensors that are supposed to keep this from happening, but, like anything else, even with every thing's operating as designed, under certain conditions evap core freeze-up happens Especially if the temperature setting is set low, the fan speed is low, and the system is set on Recirc. There is a sensor that is supposed to shut the compressor off if the evap core gets too cold, which is what I suspect happened to you. So it probably was indeed ice breaking loose from the evap core you heard. If it happens again, then have the system checked -that evap temperature sensor may be going bad.