Holy thread revive Batman!
Just wanted to tag on to the end for other's benefit. I experiened a similar issue whereby the under-bonnet fusebox was clicking furiously with the engine off for no apparent reason. There have also been random instantaneous power drops during driving which I attirbute to this issue also.
The source was the relay inside the fusebox which looks like a large cube with the number 100 on it to part number 7M0951253A (I believe that number has changed now if you buy a new part). I believe the relay to be something to do with the fuel pump but could be wrong.
I replaced this but the problem clicking still persisted. The actual cause was water getting into the fusebox and short circuiting goodness knows what. Upon inspection there was actually extensive corrosion across the entire circuit board meaning water had gotten in some time ago. Solution was to purchase a second hand fuse box (came from a Golf actually, but they're all the same luckily) for £15 and replace the circuit board bit. I've made sure the fusebox lid is on properly and water-tight.
Fingers crossed, all solved.