i have this problem with my A/C... after about 20 mins of running... this squeal noise happens from the passenger side i think. and then it happens every 10 mins or so. but only when it pumps extra gas into the Air then the squeal happens..
belt slipping (or gear that is run by belt making funny noise) when the temp sensors detect it needs to push a little more cold air in? Why not pop down to an aircon specialist ()or many) for a quote - surely they wont charge you just to have a quote done, give you an idea of what the prob may be.