Loads of information about that here (it's caused by the left/right coolant distribution balance valve getting confused), assuming the A/C compressor is working, that is. There are work-arounds, but unfortunately you will have to become a full member to read most/all of the relevant threads. Best get that sorted, Gaz.
For now, just setting the left and right side temperatures up and down to different values between Hi and Lo, may be enough to keep it happy and stop the 'di-da-whir' melody, if you are lucky.
David