My car does the same thing. If you are at a stop light and then you start going it will make a grinding noise. But if you punch it, it goes away. Its the heat shield covering the catalytic converter. Its nothing expensive or hard to fix. Its just that your catalytic converter has a 2 peice metal shell around it and its only tack welded. When a few of those tacks break loose its starts to rattle and thats what you hear. you have a few options.
1. deal with it
2. find a family member or friend who knows how to weld and tack it back together
3. take it to a shop
or just do what i did and just take the whole damn thing off. it doesnt affect it at all and it will still pass emissions
