Thanks for all the thoughts. It is a new wheel, but the problem is persistent enough and seems to be in the same area of the pedalling cycle that I'm considering it likely to be a drivetrain problem. The chain may be too tight as I was a little paranoid about dumping a chain with a slack chain, but I have about 3/8" deflection at midpath. I'll probably replace the chainring while I'm at it. I had the ring on the inside of the spider for the SS setup and have moved it back to the outside for the fixie, so it may be worn enough to be making some noise.

Thanks for all the prompt and helpful responses.

Cheers

Cam