Rattling - Fluttering - Knocking noise on hard acceleration.

Greetings Gen2 Prius owners,
I just wanted to share my experience with you all about a noise I chased for quite a long while. On hard acceleration when the engine RPMs would get over about 3000, I would hear this rattling/fluttering noise. It seemed to come from the passenger side of the engine compartment, although my wife said it sounded to her like the driver's side. Because there is no way to my knowledge to "load" the engine up like a regular car (one foot on the brake, one foot on the gas), I could not duplicate the sound standing still to try and find the location. It really didn't sound like cam or valve train noise, and a wrist pin noise typically does it all the time, even at idle.
So, I purchase one of those "Engine-ears", (a microphone with headset) to try and pinpoint it while driving. When placed at the top of the engine, it was fairly quiet. When placed down near the bell housing, I could hear it more clearly. Then it occurred to me that my batteries weren't charging like they used to, so I jumped to the conclusion that there was some kind of problem with the generator/transaxle. I got lucky and found one with half the miles of mine (the car has 260K on it) on eBay for a little over a hundred bucks and installed it.
No luck, noise still there.
So, I scratched my head, cursed a little, and decided it MUST be something coming apart in the crank case, a bearing or the like. So, I put a good used engine in it that I had on hand (I had a "spare", another very long story). Still no improvement.
Now, the car has a few other minor issues, including a reoccurring "check engine" light. The trouble code indicated a bad catalytic converter. I wasn't about to buy a $300 cat until I solved the noise problem. But, when I did the engine job, I noticed a lot of soot around the spring loaded coupler where the cat bolts to the exhaust header. Could the noise be coming from my cat all along?
Sure enough! The new cat fixed the problem, (and I found one for a hundred bucks)! The old cat was SO plugged up, that on hard acceleration, the exhaust pressure was enough to unseat the coupling against spring tension, and what I was hearing was an exhaust leak! I had replaced the engine a couple of years ago because it consumed a ton of oil, and all that burnt oil residue had plugged the cat.
I also noticed on a very short road test, that my batteries are charging up much faster, probably because the gas engine is now providing it's share of the power!
Moral of the story, if you get a code, chase it first. You may solve other problems along the way.