It does sound a bit like a possible oil related problem. Not that there is not enough oil but that it is not getting to where it needs to, especially in the head and valve assemblies, lets hope its not down at the crankshaft. On an engine that old it wouldn't be unexpected that some of the oil passages are clogged with all kinds of crud and stuff, especially if someone didn't stay right on schedule, or better yet ahead of schedule on oil and filter changes. The number one thing you can do to keep an engine running is to change that oil. The oil breaks down and starts leaving various deposits behind. It would help if you could locate the area of the rattling, if it is up top at the valves, it probably is clogging. When you rev it up, and not too sudden now, just a gradual rev up, the oil pump builds up more pressure and forces the oil through the clogged arteries. That's one reason you don't wanta punch it, the GTI could have a oil pump attack and die, or the other parts of the engine could be spinnin way fast with no oil. The oil pump itself could be wearing out and not be able to pump hard enough at low revs, especially if the arteries are clogged anyway, and the cloging could have caused the pump to wear in the first place. It's a vicious viscous circle. Even if the GTI has a real oil pressure gauge and not just one of those lights that comes on after the pressure drops and it's too late to do anything about it, the gauge may be showin enough pressure, but the pressure is high enough only because the passages are plugged and the oil isn't really going thru them. The rev forces oil thru the clog then at the 'de-rev' lets some oil just sit there and leaves behind some more deposits. Another viscous vicious circle. Now we got another vicious viscous circle. When an engine gets older and more worn some folks start using a thicker, more viscous, oil to fill in some of those worn areas, but the thicker oil can't get thru the clogged areas, so some folks use a thinner oil in their older engines but that doesn't lubricate the worn areas. Multi weight oil don't help here. Multi weights 'change' their thickness according to temperature not pressure. Additives that are supposed to make your oil 'oilier' won't help hey just make the oil thicker and you got the clogged areas problem again. You could start off by flushing the oil system. You can hire that done, or you can buy products that you can use if you know how to change your own oil and filter.. Basically they are just a solvent that you add to a warm engine's oil system, drive for a few miles then drain and change the oil. The stuff will only add about 5 bucks to the cost of an oil change so it's worth a try. The pros do have a better system that pumps a stronger solvent thru the engine, if you hire it done make sure that's what they are gonna do and not just add that Do It U seff stuff. A pro can also check the pressure of your pump. In an extreme condition there may actually be enough crud taking up space in the oil pan that when you check the oil with the dip stick there is actually less oil in there than it says. Removing and cleaning the pan cures that. But that is rare and is a lotta crud. One other thing I would check is the valve adjustment. When was the last time you did that? If the valves are a bit loose and rattling, the revving may pump enough oil up there just enough to dampen the sound. I have used that oil flush on some of my cars and I don't know if I was just fooling myself that it worked but it seemed to. There is one thing to look out for. That solvent may dissolve some of the crud that was plugging leaks and now you got a car that leaks oil. Some folks with oil leaks use a thicker oil and...hmmm......'nother one of them circles. There's an idea!Drive in a tight quick circle so the oil is forced to one side, if the rattle gets worse on the opposite side then you found your rattle, then you can.... STOP! STOP!.... all this circlin has made me viciously dizzy and flushed, it feels like somethin viscous is circlin in my stomach. Try that oil flush and let us know how it works out.

I havn't read the toehr anwser because it's huge, so I don't know if he mentioned this. But are you sure its from the engine and not possibly the exhaust? Because a reasonibly common problem can be that the heat shields around the exhaust become loose, and when accelerating (around the 2000rpm mark) it will rattle for a bit then disapear. If this sounds like your problem, take it to an exhaust place, get them to jack it up and $20 and they will weld it back into place.

I have that rattling noise on start up too in my Navara and it goes away after about 5 secs and i have noticed that the oil light stays on for about 3-4 secs then when it goes out the rattle stops. I have full oil and have recently done oil change but still got the rattle. I think it may be the oil pump