I have a 2001 Chevy P/U, 1500, 5.7 Engine, 2WD, Automatic Transmission. When it is below 40 degrees and I start it, it sounds almost like a diesel for the first 400 feet (approx.) then the sound goes away. Other than this problem it runs just fine. it doesn't make this sound when it is above 40 degrees. Any information or suggestions would be appreciated.