Ever since I replaced the cats (all 3) in my car, there’s been a metallic rattling / ringing sound, but only when I’m accelerating at ~30 MPH or more. If I’m driving slowly, or if I let off the acceleration, the rattling will go away.
Googling it, the internet said that it’s likely a loose heat shield, but I can’t find anything loose checking under my car.
What else could the rattling be?
Who replaced the cats and where did they source them from?
I’ve seen cars with old cats where the internal metal brackets that hold the substrate in place have rusted away allowing the substrate to break up. Cats can suffer internal damage when they get hit with something hard, like when you run over a rock. An engine under load will usually produce sufficient exhaust gas flow to shift the pieces around inside the cat.
No-load test for a busted cat - Rev the engine in neutral, really floor and pump the pedal to maximize the flow through the exhaust system. Perform the test when the engine has been running for while - the engine and exhaust are hot. If you hear a rasping or rattling sound coming from underneath the car as you ease off the pedal - it’s likely to be a busted Cat.