Electrics ticking

We recently had our navara fitted with twin electrics by someone our caravan retailer uses and recommends.  He used the heavy duty wiring and at first we had no problems, however on our fifth outing we noticed a ticking coming from the rear of the truck - even when not hitched up. What could it be? We haven't had any real problems other than the ATC turning red during trips but once we unplug then plug in again it's fine? Any ideas?