Posting the solution here for future searches.
The piezoelectric buzzer is located on a small board, inside the front panel of the GXT3 2000VA.
For more detail and pics, check the link below:
[link|https://www.edaboard.com/threads/identifying-disabling-piezoelectric-component-in-liebert-gxt3-ups.411409/|https://www.edaboard.com/threads/identif...|new_window=true]
I have just added a little of silicon in the hole, let it dry, and now the baby is completely silent.