A rattling noise appeared out of nowhere recently. It only happens when the car is in Drive and has been running for 5 mins or so and only at about 1300 rpms. At first i thought it might be coming from the exhaust tips, but it sounds more like it's coming from below the passenger seat, around where the cats are. It is very annoying and sounds alot like the cheap "coffee can" exhaust tips that many rice rockets have. Does anyone have any clue what's causing this?

I believe you should have a heat shield around the cat. Check to see if the cat is viberating against the heat shield.

it looks very possible... how necessary is that heat shield?

The heat shield over the cat should be part of the cat itself, attached to it. So if that's what's vibrating, it might be coming lose, or the cat might be vibrating against some part of the underside of the car.

A couple of other things to check are:

The exhaust hanger might be vibrating against the gas tank's heat shield.

The over-the-axle pipe might be vibrating against the upper Panhard Bar.

I finally took it to an exhaust shop, and they were having a hard time finding it... Finally after banging on the cats the guy said that one of the cats was going out... thats why I get this certain rattling noise. Does that sound right though?

yup that could easily be it, the inside of the cat is coming apart. It will eventually becomed cloged, but by then u will notice a loss of power and it will either smell like egg or gas.

Good luck.