The sound came later, originally it was solely an intermittent vibration through the pedals.

I've inspected all rings and the cranks for cracks, gouges and defects etc. There is nothing to see, everything meshes runs and sounds perfect when turning the cranks by hand. It's only detectable when on the bike and i'm sure you know how difficult it is to determine the location of a problem unless you've experienced it before.

I'll go and order a new middle ring and fit it as soon as possible. I don't really know what i'm doing with crank spiders etc but i'm sure i'll figure it out.

Thanks