I was out of town for 2 weeks on business and left my car in the airport parking garage. When I walked towards my car and started it remotely I noticed a loud rumbling sound. It ran for about 30 seconds, sputtered, then just shut off the engine. I then checked under the car to see if somehow someone manage to steal my catylitic converter as I've heard stories of some friends getting theirs stolen from their SUV but it seemed to be all there. I then started it up again and it was still clunking but not as made. I reved up and drove back to the office. After I came back out to drive home it did the same thing again but this time the check engine light came on. I then called and told Onstar to run the diagnostic and see if they saw any fail codes. He then told me that there was a code showing regarding emissions. The next day the light went off and then I took it up to the dealer to have then take a look. The mechanic hooked up his diagnostic tester and performed a check. He told me that whatever error I had seemed to clear itself and there wasn't a history of what the problem was. Ever since then my car seems to be running just fine. I'm just puzzled one what could be causing the problem. Any suggestions?