UPS makes clicking noises?

I just got a UPS and this is the first time i've dealt with one, and I have a question about a noise it made last nite. Anyway, I got a APC Backup UPS ES 725 for $10 at Office Depot yesterday (onsale $60 paid, w/ a $50 mail in rebate) and last night it made a click click of a Relay flipping on then back off. Im wondering if this is exactly what happend and the relay was the system going from wall power to the battery backup becuase it did not like wall power (either over or under volt, noise etc.) or is it something else.

Also, This seems to be the only real place to talk about UPS's becuase this is power related.

That is normal. It happens when you go too low voltage, or if power cuts out.

Unplug it from the wall when you have things running on it and you can hear it click.