I have that same model UPS, and also a CP850PFCLCD, and can confirm that they are in fact totally silent except during a blackout. I've used many different UPS models over the years from APC, TrippLite, and MinuteMan, and these CyberPower units are the best I've had, both in terms of features and in value for the money. They have those killer LCD displays with all the info I love, and in general look cool enough to want to leave them visible instead of hiding them. The accompanying shutdown and status software is simple and effective, and even includes a Windows 7 sidebar gadget for realtime power usage and status display on the desktop (in case you can't see the LCD from where you sit). For all of that, I'd have been willing to pay more than for the long-time standard units like from APC, but the CyberPowers top it all off by being significantly cheaper than the old standards as well. They're just that awesome, and APC should be very nervous.
That said, noise shouldn't really be an issue with any standby or line-interactive UPS unit, since they just sit there and let the wall power flow passively through them until the power glitches. There's no need for a fan or any transformer noise when idle, and I don't remember ever hearing any of my past units. The only noisy UPS units should be the online "always running from the battery" kind, which are always discharging/charging and generating heat, but you don't really need one of those for a regular PC unless your power is so awful it's glitching multiple times per day.