What would cause a knocking noise when a/c is first turned on?

Hello, This is likely your A/C compressor knocking. They normally will knock loudly at first, then quiet down after a few seconds. There may still be a tapping present, but not audible from outside of the engine bay. I would recommend having it diagnosed sooner than later. If the knocking is inside the car, let me know.

The same thing for my Lincoln MKS. But the knocking sound is inside the car at the front passenger compartment. It disappears after few knocks. Sometimes comes back but disappears again after few knocks. What could that be?

The same thing for my Lincoln MKS 2009. But the knocking sound is inside the car at the front passenger compartment. It disappears after few knocks. Sometimes comes back but disappears again after few knocks. What could that be?