I still have not figured it out. Your description sounds the same as mine and like you, I can live with it but I would really like to know what it is. I have had the car since new and I do not remember it doing this until the last year or so. Nothing has ever been done to the a/c itself--not even added any R134. I did put a new belt a few months ago. It made the same sound before and after installing the new belt. I guess it could be the clutch but I really do not know how to trouble shoot it.