I just bought a nee Sierra a year ago. I have 26,000 miles on it and yesterday i started it to leave work and there was a very loud ticking. Checked the oil and it was a quart low. Drove to napa and bought correct oil, added it and still ticking. Sounds to me like its a lifter or valve. Took it to the dealer and they listened to it. I have to take it back today after work and leave it. They want to hear what it sounds like on cold start up. All I have to say is that it wasnt making that noise before!