how can i change the natural frequencies of a centrifugal fan other then adding weights?
What about vibration isolators or spring mounts?
Your question is very broad, what data do you have to support their is a natural frequency issue? What type/design is your fan
What part of the fan has the natural frequency issue?
Fan housing/inlet cone
To shift a natural frequency you need to know which frequency you are moving, adding mass will lower the natural frequency, adding stiffness in the right locations will raise it. Other fixes are to change the operating speed or add cork/rubber/springs to dampen vibration.