I have been using an APC SUA1000 at home for about 5 years. I have used other APC devices in the past and have always been happy with their build quality and performance. However, this unit has been giving me trouble lately. It all started about 2 years ago when I came back home from a long trip to find the UPS emitting a constant, loud, excruciating sound. The beeping did not stop when the UPS was turned off or its power cord was disconnected from the outlet. The only way to stop the noise was to either disconnect the battery or braindead the UPS.
After a few e-mail exchanges and phone calls with APC customer service, I was informed that my warranty had recently expired and my only option was to send the UPS for a repair at my own cost. I decided to open the unit's case and take a look inside to see if there was anything obviously wrong with it before shipping it for repair. I disconnected the batteries, looked inside the UPS and did not notice anything obvious. Closed the box, powered up again and to my surprise, the noise was gone. It passed self test and I tested it a few times under load by disconnecting it from the outlet and it continued to provide power without failure. I was able to use the device for another 2 years or so without any trouble.
Life was good again until the unit ran out of battery recently. I ordered a replacement APC battery and installed it. As soon as I turned the unit on, I was welcomed by the good old constant buzz. Went through the same process of opening the box and disconnecting the battery a few times without any luck. I also sent some messages to APC message boards and found out that there were at least a few more people with the same problem. Their UPSs (not only SUA1000 but other models such as SUA1500, SUA2200 etc.) appear to work perfectly fine except for the annoying noise.
Finally, not having been offered a viable explanation or solution by APC, I was forced to operate on my UPS. I opened the case again, located the piezoelectric buzzer (see picture) and de-soldered it. I am sure the UPS is still trying to emit the same sound but at least I am not hearing it. It has passed self test as well as my manual tests and has been powering my computer successfully for the past couple of days.