Power lose and top end rattle

Driving along normally, power drops off and there is rattle from the the top end, If I press the throttle the engine will not rev, but if you put the clutch in and press the throttle it will, This seems to come on as soon as the engine has wormed up. I can not simulate this problem by stoping and reving the engine, if I take my foot off the throttle when the rattle starts it stops untill you press apply it again, so it seems like it is only under load that this happens. The van has 100,000 miles on the clock.