High pitch noise when car is locked

What is your question?

There is a high pitched noise when the car is turned off,locked and the alarm is set. As soon as you open the door or boot it stops and also if you press the set or unset alarm button on the key fob it stops but when it stops it soon starts again,the car works fine apart from this ?

How long has your 2009 Jaguar XKR had this problem?

A few weeks on and off