If it is a regular metallic tapping, then it is very likely to be a tappet. They are oil filled, and can occasionally leak down, and take a while to pump up again. Reasons for leaking down can be random, or it can be caused by one of the tiny oil holes clogging up.
There is a procedure on here for pumping them up - basically hold the engine a 3000 rpm or whatever for several intervals. That did nothing for me. What did work was a dose of oil flush, hold it at 2500 rpm fof 10 minutes (on ramps, mustn't load the crank with flush in it), then drop the oil and refill. The tapping was instantly improved, and vanished completely over about 5 starts.
From inside the car it is a loud tapping as you describe. With the bonnet up, you can pretty clearly pinpoint it to a bank of cylinders. Remember the rear bank is pretty low, listen through a tube to locate it clearly.