Get use to it, it's normal. The closer the subjects, the more the refocusing as the subject moves. The further away, 20 feet plus the less the refocusing. That said, I always Chose a Single focusing point, normally the center one as I shoot lots of birds in flight and use spot metering as well. If the camera is deciding which focusing points to use it will readjust focusing/chattering more often. More depending on amount of light and movement of the entire scene, not just the subject you want to focus on. The smoother you move the lens and keep on specific subject with single focus point the lesser the chattering will be. Takes some practice. An Excellent lens for the money, have taken some Great shots with the it.

Now that you have brought it up...

I do volunteer photography for the local Special Olympics organization here in Delaware, and I can tell you for sure the feeling goes both ways ;)

Even though I have the credentials displayed as an official photographer, often the shots that I am there to obtain are ruined by iPad/Iphone shooters trying to get in and get shots, some are actually so rude that they step directly in front of my camera when they can clearly see that I am taking a shot that was planned and the kids were posing for.

In my opinion, it only takes a little common sense and courtesy to notice that there are others there trying to get pictures, unfortunately it seems parents lose all common sense in their attempts to get their private pictures at public events.

I normally try to get the shot that I am there for, then move out of the way as best I can, so others can get a shot without me in the frame, however, the way some parents try to crowd in and force their way to what they consider the best spot is like a feeding frenzy, and then I can no longer get a decent shot because now someone else has grabbed the spot that I had staked out and are blocking the entire scene.

That is part of the reason only credentialed people/staff are allowed past a certain point.

This all is my pet peeve :)