Dying alarm noise help needed

[CrewOVtwo]

We've just got back home from our umpteenth trip this year and the strangest noise has started happening - like the last death throws of a duck being throttled.
It is irregular so far - never happened before, but has happened 5 times in the last hour. It lasts no longer than 2 seconds.
Our alarm has never worked since we purchased the van about 10 months ago (2nd hand), although a red led used to light up until we took the batteries out of it.
I'm going to attempt to post a picture of what I think the noise is coming from.
Hang on and pray if you have a faith.........

 

[CrewOVtwo]

Wow. It worked.
It's that black looking thingy I'm almost sure.
No power to the van .
Battery off.

 

[rich0805]

Wow. It worked.
It's that black looking thingy I'm almost sure.
No power to the van .
Battery off.

There could be a dying battery in, or near, to the object making the noise.

 

[CrewOVtwo]

The main battery in the caravan is fully charged.
Other than the two AAA batteries we took out of the alarm sensor that had the red LED light in, I don't know of any other batteries at all.
It hasn't made a sound for over 2 hours now, so I'm hoping it won't bother us again.
It due in for its service in 3 weeks so I'll point it out to them. Thanks 👍

 

[RogerL]

The main battery in the caravan is fully charged.
Other than the two AAA batteries we took out of the alarm sensor that had the red LED light in, I don't know of any other batteries at all.
It hasn't made a sound for over 2 hours now, so I'm hoping it won't bother us again.
It due in for its service in 3 weeks so I'll point it out to them. Thanks 👍

It's not unusual for alarm sirens to have their own rechargeable battery as a protection against tampering with the alarm wiring - it's possible that's dying, perhaps because you've disabled other parts of the alarm.