Have a competent automotive A/C tech listen to it. Your may just be now noticing something that has always been there. It is not unusual for the comp to be louder when it first starts turning as it is working with a much higher suction pressure. It it is quiet when running at idle and above--it is probably normal and OK. If it is failing--its too late to avoid having to flush out the line from the compressor to the condensor, the condensor, and line to the filter drier (which has to be replaced in any case.)