This is a tough one to answer. If you tell the LEO that you have a firearm in the car, like others said, you may be in for a world of problems you don't need. On the other hand, if you don't tell them, and they search your vehicle, you will get that world of problems you don't need.
A family friend who is a LEO once told me it is better to notify the cop in a polite mannor as soon as he/she approaches the car to noticy him/her that you have a firearm in the vehicle, because if for some reason, they ask to search your car and you refuse, they may interpret it as probable cause that you are hiding something, and they are gonna search anyway.
The one time I got pulled over at a safety inspection spot check, I told the cop immediately as I handed him my license and registration that I had an unloaded firearm in a locked case in my trunk, and I was carrying a licensed handgun and I possessed a valid permit to do so. I also handed him my pistol permit when I gave him my license. Very politely, he asked me where I was carrying the pistol and I told him it was in a holster on my right hip, and he instructed me to pull it out with my left hand and hand it to him, butt first, which I did. He took it and secured it in his vehicle, came back with 2 of the other officers working the detail, and asked me to step out of the car and open my trunk. I did as instructed and he then instructed me to unlock my rifle case then step to the curb. I again, did as instructed. He opened the case, saw that my AR was seperated into two halfs and had a trigger lock on it, closed the case, then the trunk. One of the cops asked what I was doing with "hardware like that" and I told him I was coming home from the range where I was training for an upcoming shooting competition, and his reply was "Oh, ok". They then did their inspection, found no violations on the vehicle, ran my license which was clean, and confirmed my permit to carry was valid, and he returned my pistol, with the clip, removed to me. No problems at all. As matter of fact, he told me that I was one of the few people that notified him they were carrying weapons and cooperated like they should have. Then I was free to go. Maybe they were just being nice, who knows. All I know is, I told them, and I cooperated with them, in return I had no problems with them.