rattling metal marble sound
 

Now i know you will all say it is detonation but the sound isnt coming from the engine. It sounds like it's coming from under me, or behind me like near the rear tire. It happens only around 2000 rpms. Could my cat be going by by?

Ps my cat is rusted through on the bottom, not sure if it is all the way through though

Probably the heatshield right by the cat. I had a similar rattle a long time ago and then got rid of it by removing the heatshield, and then put the heatshield back after the transmission tunnel right by the driver's seat started getting really hot. Nice in the winter, not so nice in the summer.

would that explain why it only happens at certain rpms?

Yes it would. BUT, when another members car was detonating, it sounded like it was coming from just rear of center and just like a shield rattle. If it happens under load and above 5000 rpm, suspect detonation. At 2000 rpm it is probably the shield if you are not under load.

yes sorry, the car was not under load, it was parked in neutral. I hear it around 2k when taking off a line too, but i suspect that it'd be the same cause. I just had my ebrake cable replaced and since it got back it's done it. Do you have to move the heatshield when doing the cable, maybe it wasnt tightened down.

I heard most of my knock coming through the AC holes in the firewall near the passenger footwell.

I agree that at 2000 and no load, it's not knock, though.

I have a name Bruce! Thank you very much!

well, its probably the heat shield considering no one on earth has ever had detonation at 30kpa.