I had the metallic whirring sound linked to engine speed and apparently its down to metal fragments shearing and causing havoc packing in the compressor slowly. Got mine done under warranty.
After having that done my condenser decided to fail and system kept losing pressure in 24-36hrs which is when they pumped in the UV tracer. Didnt use UV for the compressor issue as i think i described it well enough.
My car has just ticked over 30k miles so its certainly not wear and tear. All mine done under warranty after many trips to the dealers and calls to VW care esp for the condenser!
I think there must be an issue with compressors fitted from factory. mines 57 plate september 07 manufacture.