Thanks Grippy and ACtrafficengr, but I forwarded Nigel's post to the bicycling.com forums and the poster who had the problem never did post what it was that was causing the noise, or what he did to solve it.
I am posting this on behalf of someone at bicycling.com. I suggested that he is setting up a resonant vibration mode and to check compatibility of his chain with the cranks and also to see if the rear derailleur pulleys were tight and lubed./Bearach
"What's the deal with drivetrain noise...