Time for a new belt. Put a new one on and see what happens, but it would be an outlier for that to make a metallic tinkling sound. The belt runs all the time and the sound is when the AC is on.

 

So guys, you were right. Took the van in to the mechanic yesterday and a couple of hours later he confirmed that the compressor was the problem. It had ended up with no lubrication and through the spinning was basically eating itself and therefore causes the strain and therefore the damage on the belt also. Both need replacing and the entire system to be flushed to remove the metal particles from the system. A bit of an expensive fix, but hey, that´s what happens when you have old vehicles I suppose. Just crossing fingers that nothing else goes wrong for a while. I only bought it back in October last year, and with this fix, a previous fix, and a service (oils etc.), I have already spent nearly 2000 Euros in less than 3000Km of driving.