usually, rasp = shaking up a tin can full of angry bees, not pebbles. Usually.
I'd say (and I'm just saying) that either your old cats blew chunks (but chunks was my dog!) and they got stuck in either the resonator or the mufflers, or your mufflers are falling apart.
Open up your hood while the car is idling and throttle the car. if it's knock, it will be much louder at the engine than it would be in the car. If it's exhaust, it will be much less obvious, since the noise-producing engine is between it and you, in every way.
If it's exhaust, I would recommend a) getting some earplugs, and b) disconnecting the exhaust at the y-pipe exit ******. That will rule out the rest of the exhaust - if it still rattles, it's between the exhaust manifold and the exit ******. If it goes away, it's in the piping or mufflers.
In the navy - and I'm sure elsewhere - we call this 50/50 troubleshooting. Eliminate 50% of the system at once; it's either in that half or it's not. keep cutting the system down until you don't have the symptoms, and the one you just disconnected had the symptom. Troubleshoot that section. It's easy! Just think logically...
Mine is terrible after the catless equal length.
