had the same problem with my 54 reg Scenic, bought a couple of weeks ago. Every time I locked the car it emitted a high pitched noise/sound/whistle. While looking for the source of the noise I inadvertently leaned on the bonnet and the noise stopped. When I took my hand off again it started again. I lifted the bonnet and found there were two silver contacts on the underside of the bonnet. I guessed that one of these was being held away from the main body, then found there are two round adjustable rubber dampers on either side of the engine compartment. I screwed these down a bit (you can adjust them by hand) so that the bonnet would go further down and now the noise has stopped.
I guess the noise is to warn you that something hasn't closed properly. Worth checking the above before taking the car in to a garrage.
Hisneilness
April 2010