I dunno. It could actually be the Surly cog that's bad. I had one some time back that was machined incorrectly such that the tooth pitch was off, and the chain made all kinds of grinding sounds and I could feel roughness / grinding vibration in the pedals. I repaced it with a Dura Ace cog and problem completely went away. Since you said you bought it just last May, I'm assuming your chain has not worn out (stretched), which could also cause those symptoms.
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