A grind or rapid click when the AC kicks on. The clutch or the compressor itself.
A worn compressor clutch, low refrigerant cycling the clutch, or internal compressor wear.
If it only happens with AC on, say that at the shop. It halves the diagnosis time.
Reading about sounds only goes so far. The tuner on our home page listens in your browser and tells you what it hears. The app goes further: it names the exact fault, checks it against your vehicle, and tells you how urgent it is.