UPS Stepped Approximated Sine Wave OK for Mac Pro (Early 2008) ?

I've searched this forum and the internet but haven't yet found a definitive answer. APC tells me to ask Apple. I would rather trust the experiences of users than a non-committal verbal answer from Apple support.

The power supply in my Mac Pro (early 2008) makes a buzzing sound when plugged into my UPS (APC BR1500LCD) during a power outage - i.e. the Mac Pro makes a buzzing sound when connected to the stepped approximated sine wave battery output of the UPS.

The buzzing occurs even when the Mac Pro is powered off. I haven't tried running the Mac on the UPS's battery power for fear of damage.

The buzzing noise is as loud as the warning beeping noise emitted by the UPS when there is a blackout.

Does the buzzing indicate a problem? Does the Mac Pro's power supply accept "stepped approximated sine wave" power? Will I damage the Mac Pro by running it on such power?

Thanks to All