Two different approaches to autofocus. I have a Sigma 18mm to 50 mm f. 2.8 and the Sony 70mm to 300mm (not the kit lens).

The Sigma lens is fast but with a distinctive sound and starts focusing as soon as you turn the camera on. On the positive side it means that you can always tell when you have forgotten to turn the camera off. It might be a bother if you were trying to shoot in a quiet church for example but it is easy to get used to.

The Sony lens has a super silent autofocus motor but perhaps not quite as fast and starts focusing when you put your face up to the viewfinder.

I have yet to determine which approach works best.