Just filled my ac yesterday, as it was empty. Haven't noticed it during winter, even AC was always on and Auto.
During that time, no sound were heard from the system. But when spring came, understood, that it works empty.
Now, after filling when AC is on, randomly it sounds like different flute notes are comming out of vents or somewhere front. At the moment i push AC button of, it dissapears. Mostly it appears on idle, standing at the traffic light. While driving under torgue, it seems no additional noices appear.
Any idea what to look for? Maybe that has something to do with pressure, as empty system had no noices. Or it may be caused by bad filling?

 

Is the belt slipping? Might have a weak tensioner’