I bought a used Sigma 28mm 1.8?

So yesterday i bought a used Sigma 28mm 1.8, I got it for $150, it has a little tiny scratch in the glass, but its not noticeable in the pictures (i already took a lot of pictures with it and i didn't see anything) The only problem that when im on manual focus (i can only use manual focus since i have nikon d3100) its a little hard to focus, its smooth but it kind of gets stuck, but JUST A LITTLE and it makes a scratchy noise when im focusing, i noticed that when i bought but i didnt care for it since i felt like i was getting a good deal and i thought i could just get used it to, and i will, but could that scratchy noise be a sign of a bigger problem? Could i fix it? and when i switch it to AF (which i cant use) it doesnt make the scratchy noise.