This is NOT about changing an alarm sounder, it is about my problem with my alarm sounder that went away as mysteriously as it arrived.

In short, my S Type 2.7 twin turbo (2007) started beeping, it sounded like a quiet alarm, not shrill, just very annoying. It usually happened at night (sods law) although none of the car's lights were flashing, neither was the horn sounding. The beeping seemed to be coming from the drivers side of the engine bay (right hand drive) about the midway point. It can be stopped by unlocking the car with the remote keyfob but once relocked it sounds again within 30 mins. If left beeping, it stops for a short period then begins again. It can be fine for a few weeks, then it starts again. I was informed that this could be the alarm sounder backup.
I purchased a new replacement sounder unit and then began to look online to find a guide on how to change the unit. (I never found a complete guide)

After several weeks the problem just went away. It has been 2 years now since it happened and it has NOT returned. I still have my repacement alarm sounder just in case. *See below for suspected cause.


Battery condition

*I hadn't used the car for some time. The weather was very cold and I suspect the battery was below par. It appears that the sounder is a secondary back-up to the main alarm system and is there to prevent tampering so it figures that the reduction in battery power may be 'seen' by the alarm system as an attempt to interfere, therefore the alarm sounder is activated. I welcome any confirmation or argument on this point that may assist others to understand this potential problem.

Once I had used the car a few times, (charging the battery via the alternator) the problem simply went away and has not returned in the last 2 years.

 

A guide for replacing/disabling the cause (backup battery) is either on here or the UK forum. Generally not needed as the usual cause (of this fairly rare issue) is a flattish main battery, thus stressing the alarm's one.