My A500 power supply started to hiss and whine about 3 years ago. It was pretty quiet and high pitched at first (similar to the barely audible sound of a CRT being on). As time went on, it got much louder and I noticed that the hissing sound matched the pattern of HD usage. I suspected that maybe I was overloading, but I didn't have much attached and I read that the A500 supply should have plenty for what I was using. I started searching for a backup in case the thing died on me (it was 15 years old, after all).
Anyway, after about a year - without any change in power usage - the hissing stopped and I've been using it ever since (every single day, average of 3 hours/day).
