Mazda6S engine failure at 55K miles!!!

My Mazda6 is currently at its 55,000 miles mark. I bought it at a certified dealership when it was at its 20,000 miles mark. I have serviced it regularly at a Mazda dealership regularly and had no issues with it until an engine check light came up last month. I took it back to the Mazda dealership service and they diagnosed it to be a converter issue and as they did not have stock they ordered it and told me it was fine to drive around until then. It took two weeks for the part to arrive and in the mean time my car started to make some rattling sounds and wasnt running as smoothly. I took it back to the dealership and they fixed the part but they had no clue regarding the sound of my car. They told me to drive around and see if it goes away. On my return from the dealership the car stopped at a traffic signal, the sound became worse and the check engine light turned ON. Now they think it is an engine failure and nobody knows why??? Now it is costing me $7,000 .....I did not have an extended warranty on the car. I am screwed. I called Mazda and they offer a mere $2500 assistanship....Does anyone have any comments on this. I would appreciate if you can advise me regarding what I should be doing regarding the car, getting more help/assistanship etc.