I have a rumble when I start the car and a gass smell like a truck. I think its from my cat converter. Their were no check engine lights or anything. What's going on?

 

What kind of car? '72 Pinto? They don't have catalytic converters...

Really, if it's an Accord newer than about '98 or so, the ECU is pretty good at knowing whether the cat is good. But first you have to make sure the check-engine light works. Turn the key 'ON' without immediately going to 'START', so you can watch the CEL light up for a couple seconds.

 

open the hood and then look and listen while someone else starts the car.