[GTP Dad]

The best thing to do would be to crawl under the car and look for loose items. Sometimes heat shields on exhaust manifolds and catalytic converters come loose and make noise. This would be the place to start. Also look at the exhaust system as it is always a source of rattles. Finally there is a possibility that your cat is going bad. If they melt down and break up into chunks that will cause a rattle that you may be able to hear especially at higher RPM's.