Try seeing if the chainring is bent vertically, or offset slightly in the spider bolts
i.e. it moves slightly up and down rather than side to side like you've already ruled out.

That would make your chain slightly tighter at a certain point in the pedal stroke, and cause such a noise, perhaps.
If it's that, there is an excellent article in Sheldon's about it but i can't for the life of me find it right now. It's in the fixed section, somewhere :)

Or alternatively it could be your cog on the rear with a similar problem.