Well, the heatshield sounds most definitely like a rattle, while the A-pipe failure sounds like a grinding buzz that doesn't really taper from start to finish (It's more like it clicks on and clicks off)... it literally sounds like "mmmMMMM -BUUUUUUUUZZZ-MMMMM" when you accelerate over that RPM range. To check though, if you tap around there with some wood and not too had (As to not damage the cat), heat shields will rattle significantly, while the a-pipe wont make any noise with regular tapping.