Mine's started doing a similar thing, more like a "knock" than a "tap", doesn't sound as loud or as metallic as DrRollo's videos. I can't even hear it under the bonnet as far as I can tell so far, only when sat inside the car at the moment...
It doesn't do it when cold, only when you've driven a good few miles... then when idling in traffic or whatever you can hear it knocking, disappears as soon as you give it any revs and then sorta fades back in as the engine returns to idle. I guess from what Mike says that means it's probably the de-phaser pulley bearing?
All I want to know is really, I'm on 35700 miles, I'll be taking it in for the 36k service anyway in about a month's time, is the engine going to destroy itself in the meantime or is it perfectly fine to wait and just add it to the "list of faults" like I normally do at a service?