New one on me. Car parked up and locked on my drive and suddenly it generates a loud continuous clicking noise.
This is with everything turned off and no key in the ignition.
I investigate under the bonnet, check relays etc and the noise seems to be from the vicinity of the ABS pump. Quite loud. Under the fuse box but difficult to pinpoint exactly, even with a mechanics stethoscope.
I turn on the ignition, ABS pump starts and the clicking stops. The pump runs for maybe 35 to 40 secs before it builds pressure and cuts out. So I suspect the accumulator needs changing. It has lasted 20 years.
Car drives and brakes OK. If I lock it again it is quiet for a few hours but the clicking returns. Presumably as the brake pressure drops.
Question ... what can the clicking be with everthing turned off?

Looking at the diagram Relay 17 coil has ignition switched live on one side of the coil and is grounded through the pressure switch so shouldn't pull in without the ignition on. When it pulls in it supplies power to the pump from Maxi fuse 3. So it sounds like Relay 17 might be the one at fault. It isn't a standard relay either. Although a standard 4 pin relay will fit the socket it should be a dual contact high power relay so a standard one will burn out pretty quickly. The one you need will be black and the part number is PRC9566.

Thanks Richard. I had the fuse box lid off and the sound wasn't coming from the relays.
However, just in case, I have ordered PRC9565 from Island4x4 anyway @ £33.
It was more from underneath. I was 90% certain it was the pump but couldn't hear anything with the stethoscope.
Maybe the faulty relay is switching the pump on and off rapidly?
Have disconnected the battery in the meantime.

Relay replaced and problem solved.

I spoke a bit too soon.
Ticking off some more jobs on the car while we are on lockdown, I have just replaced my accumulator and changed the brake fluid.
Then discovered the clicking had returned !
Investigating further, I disconnected the battery and loosened the fuse box and I now think the clicking sound has been coming from the siren underneath. I have left it unplugged and there has been no repetition so far. I have also put the original relay back in. Seems I might have wasted £33 on the new relay.
Can't think what could cause it to click. Seemed to be rapid and regular, then pauses, a few slow clicks, then stops for a bit, then starts again. All with no key in the ignition. I think I have turned the alarm off in the BECM. Will need to check.

Sounds like you have the battery backed alarm sounder and the internal battery is dying.......

Unplugging the BBUS siren doesn't appear to have triggered any alarms (yet).
I read another post, Richard, where you gave the procedure for disconnecting it.

They are quite dear to buy new. Cheapest is about £100.
As a matter of interest has anybody replaced the NiCad batteries in them?

Easiest way to do it if you decide you need to keep an alarm sounder is to just connect some sort of sounder between the Purple/Yellow and Black wires in the 4 way connector to the sounder. The other two wires are to keep the internal battery charged. A cheapo piezo sound bomb (https://www.ebay.co.uk/itm/Twin-Piezo-siren-sounder-sound-bomb-for-security-alarms-110-decibels/254457190394) should wake the neighbours.....