I tend to agree with the bad converter diagnosis. I was looking at the converters together with the machanic and I don't see the shields being loose anywhere. The rattle appeared after I've been sporadically getting and clearing the error already but the frequency increased dramatically after it started. Prior to that, however, I'd noticed that it usually happens after an oil change or after pumping gas.
There are no ehaust leaks that can be detected, by a manual inspections at at least.