the easy way to check if it's your dephaser is open the bonnet, get the car up to temperature and get someone to sit in the car and rev the engine gently back and forth from 1000 to say 3500 rpm while you listen to the engine. At about 2500 rpm the tapping will stop (significantly diminish) if it's the dephaser. Also as it crosses the 2500 point if you listen carefully you can hear the dephaser switching, hard to describe but you'll know it when you hear it.
The dephaser (please don't shoot me if i've remembered this wrong) is part of the variable valve timing system VVT and basically stops the car being too sensitive to drive at low revs (around town etc) but plenty of power when you give it the boot, so basically two 'settings/profiles'. When the car is off or at idle the dephaser pulley is in one setting but when the revs reach 2,500 the dephaser switch opens and oil goes to the dephaser pulley which unlocks and does something to the camshaft which gives more power (valve overlap rings a bell but i'm not sure exactly what). When the revs drop back down below 2500 it switches back and the pulley/Camshaft returns to the Normal position.
When mine was knackered and a specialist pointed out the fact they said they'd not heard of one failing completely but that it was possible and that if one did fail properly you can probably kiss goodbye to your cam belt etc.
