Sigma 30mm internal clunk (normal or me being ignorant?)

This is the first native E mount lens I have owned, I'm used to adapting old Canon FD lenses so just a quick question.

I know that Sigma have used a new technique in having the focusing done internally but when the camera is off/lens not attached there is a slight clunk of the element moving back and forth a couple of mm, this stops when the camera is on though. Is this normal?

Thanks a lot

Sam