When the engine is running (and not with just the ignition on) I can hear a clicking like the sound of a indicator relay going on and off inside the rear interior panel near where the power socket is right at the back in the boot (trunk) area.
No indicators are switched on and no lights are on.

I do have a Towbar fitted with twin electric sockets, could it be something to do with that?
If not what could it be?

I get some minor clicking from the rear window seal at times, when engine is running due to vibration of the CRD.

Definitely not something rattling it sounds electrical

Just below the power plug do you have a pocket in the door with some mesh/netting in front of it to hold small stuff or do you have a, probably black, plastic cover? If you have the plastic cover IIRC it's hiding a remote multi-disc CD player and what you hear may be that unit cycling for some reason.

Otherwise just pop the trim off and take a look at what's in there like a relay that's cycling. Should not be too hard. You will need to remove the plastic trim strip along the bottom of the boot at the back edge of the carpet and then pull the interior trim panel loose at the back edge. Both those items are held in place by plastic friction pins that are relatively easy to pull loose although a thin 1" wide putty knife might be helpful as a prying tool.

My son's 06 had a weird rattle sort of back in that area but only when the vehicle was moving. Took forever to track it down to a loose piece of plastic inside the right rear door panel where the screw in the arm rest is located. Piece was bouncing up/down on the shaft of the screw. A bit of plastic glue and no more rattle.

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Sold to LOST member my 05 Ltd, GDE Stg II turbo + TCM tune, SunCoast TC w. Transgo kit, Steiger window regulators, Samcos, Fumoto valve, 2nd gen filter head with Lub. Spec. bleeder, Hayden clutch & 11 blade fan, inverted spare, P-1 battery, BF Goodrich Long Trail TAs, Etecno1 glow plugs, timing belt at 50K miles/8 yrs

Hi yes it is netting

Then I'd guess some relay but really have no clue what's inside back there nor why whatever it is would be clicking.

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Sold to LOST member my 05 Ltd, GDE Stg II turbo + TCM tune, SunCoast TC w. Transgo kit, Steiger window regulators, Samcos, Fumoto valve, 2nd gen filter head with Lub. Spec. bleeder, Hayden clutch & 11 blade fan, inverted spare, P-1 battery, BF Goodrich Long Trail TAs, Etecno1 glow plugs, timing belt at 50K miles/8 yrs

OK had a look and its all the wiring for my twin socket tow hitch, I have now removed one of the sockets with the relay attached as I have never used it, I guess it was power for a caravan, still got the one I need for towing my boat trailer so now all OK.