I have noticed that some times when I start up my truck in the morning, specially if it is a little colder, that I hear what sounds like a mild back fire, or a sudden "puff" sound from the exhaust. I don't know why. I took it to the dearlership and left the truck there overnight so they could start it up first thing in the morning to hear it for themselves. They heard for themselves what I was talking about but they told me that there is nothing wrong with my truck, that they hooked up the diagnostics and everything checks good that it was "normal" and that it is doing that to warm up engine faster and bring it up to operating tempratures quicker!!!
Have any of you experienced anything like this?