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I've heard 2 schools of thought on that one. The first is to simply keep your mouth shut, and if he asks answer calmly what you have, where it is and where your permit is. Keep your hands in sight. The second is to tell him right off the bat "Officer I just want to make you aware there is a firearm in the vehicle, I am licensed to carry it and just want to make you aware of the situation."
Either way I think you can expect the felony stop procedure while they secure the weapon and check your permit, so I tend to lean toward #1. If you're just getting a speeding ticket or something it is most likely to never come up, they'll never know and you're on your way in 1/4 the time without getting cuffed up, thrown on the car, etc. I guess it's a personal choice though.