I hear you... my '04 Endeavor just hit 60,000 miles is rattling the same way... I suspect the cat as well..
However, if your vehicle has less than 80,000 miles and is less than 8 years old, the dealer HAS to replace the cat free of charge under the EPA's Emissions Warranty Act of 1995.
Go to your dealer and demand they cover it.
Dealers and repair shops know about this EPA mandated emissions warranty, so if you went to the dealer originally and the service guy said you weren't covered - he sandbagged you and I'd report him. If it was a local shop, he sandbagged you by trying to keep the work and charge you for it as opposed to telling you to go to the dealer and get it replaced for free.