i own a 54 plate focus ST170, recently its started making a weird "clinking" noise like a metallic vibration from under the bonnet when driving along at low revs, i thought it might have been the manifold heat shield vibrating so i removed it and took it for a quick spin, its still there but seems quieter than before. the noise seems to only happen when driving around 30-40mph as soon as the revs pick up its gone, any ideas what it could be?