The noise you are all hearing is the security system arming. You can test this by sitting in your car after you have parked and removed the key from the ignition. Once parked, open and close your driver's door while remaining in the car. Using the key fob, lock all doors. At this point if you open a door from the inside prior to the buzzing, the alarm will not sound. Open the door after the buzzing, the alarm will sound. Definitely nothing to do with the turbo as I have an LS and hear this sound all the time.