Yet another sudden issue with the electrical system. It's a complete mess under the driver side panel!
So this is the issue: when opening any door a load alarm appears. I find that the alarm comes from a different place (or may be quieter) depending on which door you open.
I unhooked the negative battery terminal for 10 minutes, but this didn't solve the issue. When pressing the button that the door hits (so the "alarm" knows the door is closed), which stopped the noise.
Does anybody know what causes this and how I may be able to resolve this?

 

An old trick that sometimes works with alarm systems is to disconnect the batter, turn the key in the ignition to the 'on' position and then reconnect the battery. That disables (or bypasses) the alarm on some systems.

 

Thank you for the reply. Does this prevent the system from working?
I currently disconnected the buzzer of the driver door. So I found that the other doors each have a seperate buzzer, although it's a very high pitch.

 

Well, apparently the buzzer of each door is connect to the windshield heating (Is this stock??).
I suddenly rememberd that I turned them on this afternoon while driving and didn't turn them off.
Thanks for the help tho!

 

In this systems where my suggestion works, it disables the alarms completely. Sounds like you might have something different though

 

I think it's a build-in system special to the performance edition by BBS. Although it's an XJ, it's still a European XJ...
The problem is sorted, so it's all fine.