Had a quick inspect of the engine bay tonight and it seems the source of the newest rattle is the heatshield on the exhaust manifold. Its either come partially off or is really lose and vibrates against various bits of metal when the engine's running. I dont think it would be too hard a job to fix if it weren't for the layout of the Elise - its a bastard to get at it!
I've posted on the MLOC forums so, fingers crossed, someone can give me some advice. If I cant do it myself then it'll be another little job for Horizon when it goes in for a service.
Can resort to turning up the stereo for now