The autofocus motors in Sigma lenses that are not HSM tend to be noisy, but I presume yours is now more noisy than usual. With that, and not focusing properly, considered together, It seems to me the motor has failed or the lens parts have partially seized causing the motor to "struggle" to focus parts that are not moving as they should. I.e., once focused the motor will stop, but if the electronics are telling the motor the lens is not if focus, it will continue to try to achieve focus. Most likely, time for repair, and if in warranty, contact Sigma repair via Sigmaphoto.com.
Answers & Comments
The autofocus motors in Sigma lenses that are not HSM tend to be noisy, but I presume yours is now more noisy than usual. With that, and not focusing properly, considered together, It seems to me the motor has failed or the lens parts have partially seized causing the motor to "struggle" to focus parts that are not moving as they should. I.e., once focused the motor will stop, but if the electronics are telling the motor the lens is not if focus, it will continue to try to achieve focus. Most likely, time for repair, and if in warranty, contact Sigma repair via Sigmaphoto.com.
You dont say what fit your lens is have you trided to use it in manual focus if its ok then it not the lens and may be the camera