Did dealership have an obligation on this sale that didn't happen?

This is about dealerships and their obligations. This past summer I purchased a 2010 Jeep Patriot from a very large dealership. it is rare for them to have a car "this old" on their lot. (By the way, I have purchased cars before from small lots too with one owner.) Not a new car obviously but the body was and still is in amazingly good condition and the original miles on it for a 2010 were low (60,000 at time of purchase). I've had it for about 6 months and have found out there is a good chance the catalytic converter and any parts associated with installing it may have to be replaced. I am being quoted in excess of $2000 at various places. The warranty on the car is over and I could not get an extended warranty on it at time of sale because of it's age. It has the original catalytic converter and related parts. The dealership does their own inspections but the car was inspected and had a new inspection sticker on it at the time of sale. I thought the exhaust sounded a little loud when I took it for a test drive but the vehicle drove alright. So I bought it. Since buying it the exhaust got louder and that's when I started to have it checked out at a couple of places. I decided to take it to the dealership this past week to their service area look at the exhaust in addition to the other places I had check it out. They said they are not going to pay to have the catalytic converter and resonator replaced "if that's why you brought it here" and that to replace them is going to be on me. My question is, shouldn't they have known at the time of sale that this vehicle had an exhaust situation going on? Was it "dishonest" or "fraudulent" for them to sell it? I know those are harsh words, but just asking. I can't say they were dishonest or fraudulent. The dealership claims a few employees drove the vehicle when moving it, etc. when they first got it and no one said anything to them about any exhaust sounds. I am asking myself if they admit now it needs work, how could they have not known 6 months ago. I guess I have a normal skepticism about if the car needs this kind of work, didn't it need this work on the day it was sold? Does a dealership have an obligation to figure this out before selling it? In excess of 2g is alot of money to fix a car I haven't had that long! Thanks for any input.