Over this weekend I have noticed a possible problem with my aircon / climate control. After say 20mins driving as I go up through the gears I get a sort of bearing humming from the aircon at the top of the revs just before i change gear. If I switch the aircon off i dont get the noise. In top gear I get this at about 60 -70 but can drive through the noise and it goes away past 70 (not that I would ever go that fast It is there sometimes and sometimes not. I also note that the aircon seems to be drawing more power from the engine and coming to a junction etc the revs drop to say 400rpm (then climb up to 600 - 800 rpm) and often the car will stall. On looking under the bonnet I noticed a few drops of liqard on the engine bay tray and touching the aircon pipe it was wet but I think this was condensation. The air coming out the vents is cold. Any thoughts.