definitely not yelp. Only humans and animals 'yelp'. There are generally two sounds made by a car when it is unlocked remotely. The first is a 'click' which is the actual sound of the door lock releasing. The other is a 'beep' which is an electronic sound the car makes to alert you to the fact that the car is unlocking (or, locking).

Its kind of a high pitched like "Beep beep". Not like a car horn, but almost like a high pitched button.