Intermittant relay juderring on Vectra

My Vectra (Elite) has taken to making a juddering sound every 5mins or
so. Only recently noticed it but may have been doing it since new. It
happens when the car ignition is off i.e. the car is standing. Appears
to do it all night long, but haven't had any problems with battery
charge even when left for several days. Doesn't matter if car locked or
open, but comes on sometimes as if waking up when you open door etc,
not always though.

The sound lasts approx 4seconds, sounds like a stuck relay (solenoid).
Clicks about 6 clicks a second. Coming from behind the dash (passenger
side). "Didle-ly-didely Didle-ly-didely Didle-ly-didely Dum" (6/8
Molto Allego).

Same thing happens when driving and you turn on/off AC manually.

Sounds like somekind of management/houskeeping function, no failure
indicators though.Is this normal?