That cog looks like it has very little wear. I'm curious how the back of it looks, are the teeth silver from the black coating wearing off?

Despite the cog being used, it could just be that the cog is less worn than your chain and the tiny difference in spacing is making the noise. Most times when you introduce a new part (cog, chain or chainring) there is a break-in period where there will be some noise, but it'll quiet down after you put in some miles.