I hear a random hard metallic click, bit like a hammer striking steel, at the front of the engine which I assume to be the VVT solenoid(s). My question is should they make this noise? If not what's likely to be amiss?

 

Shine a bright light on the accessory drive belt tensioner and observe whether it snaps back and forth when the hammer noise occurs. If it is, your belt idler pulley bearing is probably bad, or the tensioner idler pulley bearing is bad.