Steve40th

My 2001 CRV with 197000 miles is rattling upon start up. I checked the exhaust and its fairly tight, the heat shield on the converter is tight. I have no codes, even though check engine light is on. I had two places scan it with handheld, and my own and nothing. I had a weird chassis code a long time ago, but no number correlated to anything.
When I initially start vehicle, cold it rattles till warm. Then I have to lightly rev it up and let it idle and as it is coming down it sounds like rocks in the cat. I am assuming cat is bad. The shield is tight, no issues there.
So, thoughts? Any other known rattle areas?
If I were to do the cat, what gasket goes between back of cat and exhaust. There are two listed, one is a standard Triangle exhaust gasket walker sells with their cat, and another round one that goes in the round section of cat in the back. I know there is a donut style for the front of the cat to pipe coming down from exhaust manifold,but the rear one is listed on rock auto as BOSAL 256792, and the Walker triangle is 31383.
Well, let me know. Looks like an easy swap, as bolts arent real bad for the mileage..Rears are a little rusty, fronts are solid.