This was originally posted on APC forums on 10/17/2010
I've had a USP ES500 for about two years. Today around eight in the morning, it began to make a consistent high pitched whining sound. Not a vibration or a quiet whine, one that can be heard from a few rooms away. The whine stops when the power it shut off, but starts again after a few seconds when turned back on. My computer, connected through the back up, works even when the USP ES 500 is on, but other things such as the speakers do not.