You have a PC plugged into an Uninterpretible Power Supply.
When you turn on the UPS, something inside the system case is making a noise.
Is that correct?
What kind of sound is it? Vibration, grinding, beeping, tone?
Does it go away over time? If you kick it? (joking - sort of ... I had a bad fan in one of my machines that would squeal until I tapped on the case)
*** I don't recommend kicking or hitting your PC ***
I would go a bit further.... Has this just started happenng or has it happened for a while. My UPS has a five second or so stabilising time on switch on during which it does make a noise rather like a squeal. It also does the same if the power goes down, just to let you know that it is taking over and then runs quiet. It has always done this and I believe in my case it is meant to do it!
Secondly you say the CPU (central processing unit ie the microprocessor do you mean in the computer?). Finally most UPS (uninterruptable power supplies) have waht is known as switching transistors as well as a far. Both can make the case rattle against something it is close to hence techs heaven cure by kicking it!
I hope you realize that though we have injected a bit of humor, we are seriously interested in helping you resolve your issue. Asking clarifying questions is the first step. I'm watching for your response.