I've just bought an used AF-S 17-35 f/2.8 len. It is two years old, but it has been used very few by the original owner. In autofocus mode, when the focal ring rotates, I hear a hiss of medium intensity coming from the len. It is noisier than my older AF lens, without ring motor! I tried other AF-S lens, like the AF-S 80-200 f/2.8, but they are very silent. Is a medium hiss normal for this kind of len?