When I'm driving my car in the morning between 40-50mph I hear a rumbling beneath my seat. I also hear it when I'm on the highway cruising at 80mph. It goes away after 15 minutes. I scraped the exhaust/cat a few months ago. A few days later my check engine light came on for bank 1 sensor 1 for the cat. I went to the dealership to have my A/F mixture setting reset but it came back on. I assumed I had an exhaust leak. I had it checked at a performance shop and they said was no leak. My FI exhaust should be arriving in a few weeks. Should I purchase another pair of HFCs?