From personal experience things using propane refrigerant seem to make a high pitched noise, I'm not sure if that's a property of the refrigerant or how devices using it are built.
There's no "reason" propane has to be louder but most propane refrigeration systems are used in industrial or cost-conscious applications where making the equipment quiet is less of a priority. When designing pumps it takes a non-trivial amount of engineering effort to ensure that the bulk of the noises emitted are above/below human hearing range.