A couple months ago I picked up a low rpm rattle, figured from experience it was either a loose heatshield or a cat going bad. Got underneath, the shields & cats are fine...but above the passenger side cat there are two heatshields which are more or less fastened together with tabs cut into the shields. It was this tabbed joint which was buzzing. My fix was to press them together with a couple of clamp-type paperclips that come in an array of sizes.
That seems to have stopped the noise.