sounds like a tractor

[aaand]

what happens if it goes???

[Si]

The Big end and Main end Bearings tend to rumble, like a wheel bearing style noise. Tappets tend to 'tap'.
Like HC-animal said, it could be Exhaust manifold or down pipe? Open bonnet, when engines running listen to where abouts noise is coming from..if it back of engine then it could be exhaust, if its bottom of engine its likely to be big ends, and tappets and inectors are on top of engine.

[aaand]

convinced its the tappets  can i make em sound  a bit queter for the time being, 

[Si]

Check your oil level, sounds obvious but always best to check that first! 
If thats fine then you can get some oil treatment, not 100% sure what its called tho!   

[aaand]

cheers mate, hope this works

[Organisys]

Tappets a reasonably straight forward job, doesn't take too long. Parts about £80 If I remember right 

[Judderi]

If its the tappets then the id do the following.

Dump all the oil and throw away the oil filter.

Replace the oil filter with a genuine vw one as they have a non return valve and fill up with vw syntha silver oil

If that doesent help then there is a tappet treatment fluid you add to the oil that can help. I think its made by wyns.

But also i had really noisy tappets until i got my timing corrected and now they are silent. Worth getting that checked too at a garage if like me you have never done it yourself.