Why does it sound like my engine is rattling when I turn my AC on

Hello, sounds like when the air conditioning system is turned in, it is putting added stress and pressure on the pulley system - including the drive belt, tensioner pulleys and idler pulleys. Also, the compressor itself has a pulley that could be making noise. This could be due to wear of one or more components, or possible failure -- or pending failure - of the compressor itself. I suggest some hands on testing - either at the dealership again, showing them exactly what you're experiencing, or at another repair shop near you. This is important to check, because the noise could be a warning of worse things to occur.