when you have your ac systems topped up by professionals they add a certain amount of oil in to the system too there will be a metal plate under the bonnet that tells you how much oil should be added when the systems are re-charged. if you have your ac topped up with the throw away cant you are not replacing the oil in the system if your compressor is making a noise i would recomend you have a full recovery done then a top up with new gas and oil replacement.