Could be the idler pulley as you mentioned but mine turned out to be the material in the precats had broken up and was rattling around inside the manifold. Try a mechanics stethescope to help isolate the noise. You can hold the blade of a large screwdriver to different parts of the engine and 'listen' to the handle to hear where a rattle or rub are coming from. (Try the rear exhaust manifold
) The waterpump is on the other side of the engine so that is probably not the source of the noise. A 3/8" breaker bar (iirc) should fit the belt tensioner to make changing easier.
