Bent chainring or cranks?

lightly grease the chainring bolt threads?

Tried it - made no difference.

mine makes alittle bit of noise but i think that it could be abit of chain stretch... whats your tension like?

I don't think it's chain stretch. The chain is only a couple of months old. Tension is spot on I reckon - 1/2" vertical play.

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.

I don't reckon it's vertical because when the bike is upside down, if you look down from above, i.e. in the plane of the chain ring, at the chain you can see the part of the chain on the bottom of the chain ring (nearest you) move out one way whilst the other part of the chain on the top (nearest the ground) move out the other way.

I'm pretty sure it's not the cog. EAI on Goldtec, and the chain and cog move round smoothly there, it's between the cog and chainring that you see the chain move laterally.

my reply

"I've watched the cranks go round, and I don't think it's them. I put a bit of blue tack between the chain stays and inside of the spider, and it doesn't move much. Perhaps 0.5 mm? Next to nothing; probably 0.5mm because of me moving rather than the cranks relative to the frame."

thats not even worth mentioning

I know... that's what's puzzling me... When it's all together, you can visibly see the chain move laterally.

Loose chainring bolt(s) ?

After you took the chainring off for inspection - did you put it back on to see if the problem was still there ?

Nope, not chain ring bolts. They were tight when I took it off for the first time. I put it back on yesterday evening, with light grease, and it made no difference.

This is so strange :S Thanks to all your replies so far.