This is a problem i had on my 2.0 gt mk5 golf, which is now sorted.
It is caused by the tensioner on the cam chain.
Basically the tensioner is oil fed, So as the engine is started oil pressure is fed to the tensioner so it adjusts the chain tension.
simple enough you might think, But inside the tensioner is a small assister spring. This is designed to take up the slack in the chain and stop the rattle on start up.
So what i am saying is the tensioner becomes weak and allows slack in the chain, which causes this noise on start up.
I decided to rectify this as it sounds horrible and done some research (Like you all have) and found the usual.
I then spoke to my local vw master tech who suggested I put a 5w40 grade oil in the car (standard time and distance spec oil) instead of long life as longlife is very thin. I tried this and it helped a bit but no cure.
I then decided to go down the replacement route. A long story short my mechanic and me took the chain cover off to check it out and found the timing was half a tooth out on one of the cams. so we ended up changing the chain, tensioner, and cam adjuster (not cheap).
Once we fitted these parts and put it back to togather i took for a run and all i can say is what a difference. The noise on start up had gone, in fact the engine was a lot quiter all round. It is like driving a new car and even felt quicker on acceleration!!
To be honest i didnt think these parts were that bad really and if i was doing a head gasket or something i wouldnt of looked twice at them but i thought i would give it a go and it paid off for once.
Hope this helps some people and sorry for the bad spelling.