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Unfortunalty that is the nature of this particular lens. Its Noisy and is slow to lock on a focus. the canon version 75-300 USM is a great deal better and if you try it out in a shop you will see it works miles better but its probably double the price. I use to show customers both the sigma and canon lens and 9/10 times they would go for the canon.
However in good lighting conditions (mid day sun) the lens should be ok. not great but ok
if your looking to get rid of the lens after reading this but dont want to spend much more the tamron equivalent is a better bet still wont beat the canon though.
How new is the lens? In or out of warranty? If out or warranty most likely solution is to find a Sigma authorized repair shop because your description sounds like the auto-focus motor gear has failed. If in warranty, check out Sigmaphoto.com for repair sites and also download the SIGMA_SERVICE_REQUEST.PDF. I've only had to deal with Sigma once on warranty matter and they were very helpful, provided UPS authorization for shipping on their nickel.
make sure that your contacts are clean, try some alcohal and that the magnet on the wheel is at the right angle. Your manual should tell you how to program it for km/h, don't forget you changed batteries.
All auto-focus lens have some sort of 'chip' inside .. Sigma 'reversed engineered' Canon's chip - however every time Canon brings out a new camera there is a chance the Sigma lens chip won't work (this is why you see lots of Sigma lens on eBay :-) )
IF you can prove you purchased your lens direct from an authorised Sigma dealer, Sigma will update the chip for free .. otherwise they charge about £35 .. you pay postage in both cases ..
I have the same problem. So far I've contacted Sigma (I believe this is the company that manufacturers the Quantaray lens for Ritz) and they requested I send them a photo of the lens. I just googled sigma lens to find sigma support. I will update you all with any response I may get.