I was woundering if anyone has ran into this problem i have older sigma 400mm prime auto focus lens. I bought a 1.4 SIGMA telecoverter to use on the lens i read in the chart that it would work but only on manual so i said ok so i put the thing on and it worked fine but when i took the teleconverter off and put the 400mm on and switch on the Autofocus on i started to get this strange grinding noise and now the thing will not auto focus right but works in manual and sticks. It has made me upset to think that i follow directions and the thing screws up my lens. Woundering if anyone has had this problem or any other problems with this lens and its gears.