discovery 2 v8 tapping noise increases with engine speed

Hi my 1999 4.0 v8 discovery 2 has a tapping noise that comes from the r/h bank as you stand facing the engine with the bonnet up, you can distinctly hear it coming from just under the r/h rocker cover at the back.
The noise is not there when ticking over and it is not there if you gently rev her but if you blip the throttle it is tapping,
Then when driving the car if you are always very gentle with the throttle its ok but if you have to accelerate a bit harder say going up a hill or pulling away quicker than normal or putting your foot down a little in any gear the tapping noise is there and seems to be louder the more you increase throttle load, as soon as you ease of the pedal it goes away.
Does anyone have any ideas
Tappets? rocker shaft? camshaft? fooked?
Thanks matt

Thats the usual suspects, but I assume the engine is running ok, cam wear can be heard all the time, your tappets are self adjusting, slipped liner is the worst case if your engine has overheated at any time.
A blown manifold to down pipe gasket will give a tapping sound but that's more like a mechine gun when the engine is under load ie. accelerating or going up, as I found out once.

Check all exhaust manifold bolts too before you start opening it up.

I did read on a post on another forum but for the life of me cant find it now, a guy described identical symptoms and he checked exhaust but it turned out it was actually the head gasket blowing from a cylinder to the outside world but it did not show up on his compression test.
Other posts i have read say it could be a slipped liner.
I really do not want to tear it apart unnecessarily.