Noisy Climacontrol

[UXBY]

I have just bought a 2004 130BHP TDI Passat Estate, it's covered 102,000 miles with full service history, my problem is when I use the climacontrol it makes a really bad noise, the best way to describe the sound is, it sounds like the header tank is boiling, as soon as you turn the climacontrol off, the noise stops, so have any of you guys got any ideas of what the problem may be , cost to repair etc
Thanks in advance
Geoff