Dont rule out the tensioner pulley they are renown for failing, mine is going and is making a similar noise to what your describing, sort of a metallic rattle on idle, the pulley assembly sits at the back of the engine, on the lower area of the main belt, its hard to pick out the noise looking over the engine with an open bonnet as all the other noise drowns it out, but if you leave the bonnet shut the noise is noticeably louder near the rear of the front wheel arch, coincidently the location and access point of the pulley.
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