On a trip to Loch Fyne this weekend the bike developed a strange whirring metallic rattle from the right handside of the bottom end of the engine. This was a kind of hollow rattle but was not rythmical (hard to describe really) and kicked in more when I blipped the throttle. It gradually got worse and then mysteriously disappeared again following my 500 mile trip home. It has now disappeared at idle but happens sometimes when the engine is revved. Could this be something to do with the starter clutch? If the engine doesn't fire it makes the horrible clanking noise but it's always done this and so far I've ignored it. The bike is performing fine. Any ideas before I begin the stripdown?