Tappets , Hydraulic lifters suprise

[Phillydra]

So started my van yesterday and all of a sudden it got the loudest tapping noise, I've looked at a few posts on here and its obviously the tappets, but why all of a sudden? it was fine the day before and later on in ther day when i tried it agin it was nice and quiet , 

Its usually really quiet and the mechanic at MOT even commented on it the other day said he thought he had stalled it and how nice it ran !!  thanks jinxed it 

here's a video of the horrible racket   
http://imgur.com/Tk9Gdjs

why from nothing to this really is my main question ? 

[Mocki]

It’s a lottery ! 
you have a one in four chance of it stopping on the one lifter that is draining down under pressure .... it may rattle three more times in a row if your luck is like mine ....

[Phillydra]

it sounds distinctly like it's coming from the left side, 

it's just a case of tightening them like this?

http://www.youtube.com/watch?v=2vkjuQ-A2QY