clicking noise from back-ups

Also plugged in are the monitor, dsl modem, usb hub, external disk drive, and printer. The last two are usually turned off and none are plugged into the "controlled by master" outlets. If the mini is plugged directly into the wall outlet there is no clicking noise. While power often goes out in our area during storms, there haven't been any problems in the two weeks that I've been using the new mini. The two clicks in succession do sound like something being switched on and off. This morning something different happened. At one point there was a slight delay between the two clicks and a message appeared saying that "your computer is now running on battery power, save any changes and shut down as soon as possible" (or something similar).

Since this only started when I switched from the old G5 (with a grounded 3 prong plug on power cable, and running Tiger) to the new mini (with 2 prong plug on power cord and running Mountain Lion) it seems like it must have something to do with how the Back-UPS is interacting with the computer (which it is connected to with a usb cable in addition to the power cord connection.)

I just disconnected the usb cable from the computer and am still hearing the clicks. I'm also still hearing them in response to what I'm doing on the computer (opening or closing something, etc.) in addition to at random intervals. The only thing that changed when I unplugged the usb connection was that the energy saver display in system preferences no longer shows a tab for the UPS.

I mentioned in my original message that the clicks sometimes occur even when the computer is asleep and that this is followed by the sound of the fan running (I assume) for a few seconds but without the monitor waking up. However sometimes the monitor does wake up. (This happened twice, but both occurrences were before I unplugged the usb cord so I don't know if that is a factor.)

The clicking noises are definitely coming from the UPS which is located a few feet away from the mini. (As previously mentioned the noises stop if I just plug the mini directly into the wall outlet; but this isn't a permanent solution since it leaves me vulnerable to outages or surges.)

Do you have any suggestions as to how I could test the PSU on the Mini?

I'm not aware of a reduced power setting. I couldn't find anything in system preferences or in help that would relate to this. However it seems like the power would be reduced when the computer is asleep and I'm still getting occasional clicks when it's asleep as previously noted.

I put the computer to sleep using the command under the apple menu at the top left corner of screen. When the clicking occurs during sleep I usually hear what sounds like the fan running inside the mini but the monitor screen only woke up on those two occasions. I would guess that the fan coming on would be in response to the hard drive waking but don't know this. In the Energy Saver panel in System Preferences there are two tabs, one labeled power and one labeled UPS. Under both tabs there are boxes labeled "put hard disk(s) to sleep when possible." I left these in the default settings which are unchecked under "Power" and checked under "UPS."

BTW, I really appreciate all of the thought that you are putting into this! I'm just surprised that we haven't heard from anyone else experiencing the same issue and that I couldn't find any other threads related to this (although there are a few related to other types of clicking noises, as well as buzzing noises related to ups when in battery mode.)

I checked that box but wasn't around the sleeping computer long enough to tell if it made a difference. I plugged in a lamp with a dimmer and turned it up and down with no effect. Then I had an idea. I'm going to wait a couple of days before marking this as definitely solved, but here's what happened:

The UPS has three outlets marked "controlled by master." Anything plugged into these gets it's power turned off when the computer is sleeping. I had long ago moved the peripherals to the other outlets because I didn't like having my computer and monitor wake up whenever I turned on the printer just to use the copy function. Near the power button on the UPS is another smaller button with a graphic symbol. If you hold this down for a few seconds it turns off the "master enable" function. After I did this the clicking stopped.

This still leaves some unanswered (though no longer important) questions: Why was this an issue with the mini but not with the G5? Why was I getting clicking noises in response to doing things on the computer even with the usb connection to the UPS disconnected? Why was I also getting them even when the computer was asleep? One possible factor is the usb hub. The old one was on a pci card, while the new one is external with a power cord connected to the UPS. Another factor may be the ungrounded power plug on the mini vs. the grounded one on the G5. At any rate it looks like the problem is solved, at least for now.