Im glad I found this thread. Ive been running a 18T surly cog/sram pc-1 combo for years with no noise issues. I recently switched to wipperman chain and now I get a ton of chain noise off the rear cog. I didnt even think about the cog. If I flip the bike over and spin the cranks, you can tell that the noise is coming from the rear.

I can tell you that surly cogs are quiet with SRAM chains. Dont throw out the surly cogs, just run a PC-1 with it. If you have wipperman expect noise. The wipperman side plates are much larger than the PC-1 I guess that is why its noisy with the square teeth....

On another note, my surly 18 tooth cog has squared off teeth, my 19T surly is more pointy - go figure.... Might take a dremel to round off the corners of the square tooth. My Euro asia 17t has pointy teeth too..