Sorry to necro an old thread but I had my air con on for the first time this afternoon and, after an hour or so, it started to make a noise. Definitely the air con because it stops if I switch it off (and leave the fan on). Now it does it after a few seconds when I switch it on.
Not sure I'd call it a bubbling noise - more high-pitched as if the fan bearing was squeaking a bit. It did it more when I came to a halt and was idling, but that might just be that it's easier to hear when the engine's quiet.