I would put money on starter motor mate very common on some of the vag engines, both of my previous MK4 Golfs did it, my first one did it every now and then mostly when cold this was a 1.4 16V engine and my last one which was the 1.8 20V non-turbo engine did it every time it was started pretty much and was very annoying did not make a single difference to the car, apparantly the armitures etc. get filthy over time inside the motor and they begin to make noise, can be cleaned out and it will probably solve it for a while but it will come back and the only true way to get rid is to get a new starter mate, thats my estimation anyway I know it is a very common fault on them.