Anybody else have a cog that clicks as the chain rolls over it?

I've been working on this for weeks... my chainline is now within 1/2 mm of perfect. As the chain rolls over the cog it clicks rather loudly, at least as noisy as my geared bikes would be. It's a SOMA 15T cog, KMC chain, Shimano 39T chainring. The chainring *does not* make this clicking sound, unlike the cog, which makes me think it's not the chainline. I've tried cleaning, lubricating, changing chainrings, changed the bottom bracket, etc.

Is this normal with a Soma cog? Anything else I should try? This is my first fixie, so I'm not really sure how loud the drivetrain should be, but I was expecting it to be nearly silent.