Could be overtight, loose or worn belts. It could also be the water pump or alternator bearings, or possibly the timing belt tensioner

First step is to remove the alternator belt and run the engine briefly to see if the noise goes, and same with the PAS belt if you have PAS. If neither of these things fix it, then replace the timing belt tensioner ASAP, dead cheap from
http://www.vwspares.co.ukIf the noise does go then replace both belts, and if that still dont fix it then replace the waterpump, then the alternator. If it STILL doesn't fix it then I'm out of ideas
