Springy Rattling noise on new bike I have recently bought a boardman pro 29er from Halfords (cycle to work).

I like it and I'm enjoying riding it for the most part however there is a noise.

Whenever I'm in a high gear (largest front cog, smallest back cog) and I'm at cruising speed, pedals moving but not too much force being applied I start to hear an intermittent rattly/springy sound. It's a hard noise to explain. It almost sounds like there is a loose spring tinkling against a spoke. It's a really difficult noise to pin point.

I've wiggled and shook everything I can get my finger on and cannot see anything loose. The only thing I noticed was the dérailleur clutch was set to the off position.

I plan on taking it back to halfords to fix after the weekend but I just wondered if anyone had any idea what this might be.
So eventually it seems I figured it out!

The chain was indeed vibrating like a guitar string at speed. This was caused by a couple of rear gears ever so slightly out of alignment. They shifted ok but I could see the chain was marginally catching on some rear cogs. 

A small adjustment of the gears with the dial on the gear shift and a good grease of the chain and it was running silent :)