KA24DE- Hollow cat makes worst noise ever?

[Kingtal0n]

I was diagnosing an odd noise coming from a KA24DE (1998) engine. At least, the noise seemed to be coming from the engine. It sounded like pinging, and it was load related, that is, the more you step down, the louder it got.

One idea was ... carbon buildup causing the engine to ping. 2 gallons of water later and 400 miles nothing had changed, but the pistons looked cleaner at least

I also thought it could be the cat, maybe the 15 year old was clogged or broken inside. So i removed the exhaust system and inspection revealed... somebody had hollowed the cat out already. NOT thinking anything of it (how could an empty cat cause such a load related noise???) I put it all back up and kept thinking of other possibilities. I wrote down a few suspected... broken flexplate, something bad in the ignition, something in the head, etc...

But you know how they say try the simplest things first? So I figure what the heck, lets just replace the hollow cat with a $13 test pipe from ebay.

And Walaa... Noise is gone.

Now, I have hollowed out cats before in my day, on a Camaro with a V8 mind you- and never heard of it causing a pinging type of noise. Was wondering what others had experienced in this way?