Every time I lock my Vantage, I hear a noise come from the car about 15-30 seconds after. It almost sounds like an impact wrench (but not as loud obviously).

Anybody know what this is? It's done it since I've owned it. Sportshift model.

Double locking. 25 seconds after initial lock (deadlock) for additional security, the doors cannot be opened from the inside handle. Should just be and audible click, unless there is gear damage. New door latch required.

Makes sense. Mine sounds exactly like this:
I'm sure they used the same Volvo actuator.

Same sound as in the video from my V8V.