Exhaust Rattle?

Put it on jack stands and have a buddy wiggle the tailpipe while you're under there and see where it's hitting.

they usually make contact under load on the passenger side above the axle. I agree with barnaby, get the car up and you can see where it is hitting.

Mine doesn't rattle, but gives a solid 'tink' when hitting a bump too fast/too deep/too hard. It sounds like the axle lifting up and hitting the pipe, but that's expected, especially with a lowered car.
If your issue is more of a rattle, jack it and shake it as mentioned in the above posts.

stprorolla49:

The exhaust has a lot of adjustment options. You can lengthen or shorten the gaps at the axle by loosening the tailpipe-to-muffler joint(s). You can rotate the tailpipes (axially [clockwise/counterclockwise] as you look down the pipe. There is not much clearance between the pipe and the corner of the gas tank. Check the U-bolts because they could be contacting the body or the driveshaft. You can rotate the threaded ends away from the point of contact.

As the others have said, put the rear up on a ramp so the suspension is loaded and get under the car and use the heel of your palm to whack the system so you can locate the contact point. It is a pain but it will be worth it.

Last thing, check the covers on your cats. If a weld has broken then the cover will rattle at certain RPM and that will drive you nuts.

HTH,

Chris