Q: Rattling sound from engine when AC unit is turned on

It sounds like you may have a faulty AC Compressor or potentially a leak in the AC lines somewhere. Car air conditioners work on a simple principle of heat transfer, relying on the fact that heat will always transfer from a warmer to a cooler substance. The A/C uses a compressor to turn a refrigerant gas, usually Freon, into a liquid concentrating its heat. In this state, it passes through conductive pipes in a condenser to dissipate the accumulated heat. As the liquid leaves the condenser and returns to its natural gaseous state it becomes very cold. The A/C system pumps this cooled refrigerant through a series of very thin tubes while blowing warm air over them to cool it before it enters the cabin of the vehicle. The heat from the air is absorbed through the tubes by the Freon gas, which returns to the compressor to begin the cycle again. I would suggest having a professional from YourMechanic come to your location to take a look at your vehicle.