Definitely should not be doing that. I just picked up mine yesterday for the same issue. They replaced the left side cat and I have no additional noises or anything unusual. I do smell a little more like burned clutch odor. But assume that's burn off from the new cat or maybe any sealant they used.
It's possible that one of your downpips/catalytic converter wasn't tightened down correctly and is leaking near the flange that mounts to the engine? if they didn't road test it, they might not notice the leak like you hear under heavy load.
Have you seen how the cats are removed and replaced on the ST? It's not a simple exhaust job like mid pipes or cat-back. They loosen and drop significant parts like sub frame, transmission mounts to get out the cats.