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 i'm 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 i'm focusing, i noticed that when i bought but i didn't 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 can't use) it doesn't make the scratchy noise.

Difficult to judge from a distance. Sigma lenses are often a bit loose in the manual focus

Call Sigma and ask them.

It sounds like you have more problems than just a scratched lens (a really good reason for someone to use a UV filter as protection against sticky fingers, scratches and blunt force trauma to the lenses front element).

That scratch noise is in the lens barrel and can only get worse unless you have it repaired. Having it and the lens scratch repaired may cost more than a new lens. Were you able to test the lens before buying?

If you use the electronic rangefinder in the camera, you should get excellent focus

Sigma's can be stiff focusing. It should be fluid to move the focus ring and not 'sticky' or HARD to turn. It shouldn't have any 'hang ups' any where in the turning.
Make sure it is in manual focus. I believe that the switch is on the barrel in that one? It slides forward for AUTO and back for Manual. AYou could be hearing the af mechanisms if you are in auto focus and manually focusing.

If you are in Manual make sure it is 100% IN manual. Those lenses that the barrel switch moves back and forth are sometimes very easy to get half way in/out of auto.

If that is not the answer (and I suspect it isn't) it sounds like the lens has been dropped. The noise you are describing along with the scratch on the lens kinda makes me think it has. In that case I'd guess that the barrel is damaged or slightly bent. If there's even a very SLIGHT (unnoticeable damage) to the barrel of a lens it will grind and be hard to focus. Yes, it is a bigger problem then. You can still use it, however eventually the threads will strip out and it won't focus at all. If the damage is slight it may be a lot of wear and use before that happens. If it is more serious damage it may not hold out so long. You can take it to any camera repair shop to be looked at and estimated, cleaned and repaired. You can also contact Sigma and send it to them for repair estimating. If there's no damage done to the lens at least you know!