consider me to be (at least) the fourth user who wants to know the cog manufacturer.

also, ken, when you say "rhythmically" i assume you are referring to a "periodic" chain noise. i'm thinking you could tie it to either the chainring or the cog based on the period, relative to turning the cranks. if the noise (or loosening-tightening) is "rhythmically" synchronized with turning the cranks, then it really has to be related to the chainring, don't you think? alternatively, if the period is synchronous with the wheel rotation, then yes it pretty much has to be the cog. if it isn't really in sync with either, though, then the problem is likely with both the ring and the cog. at least that's what i think.