If the power supply fan suddenly started acting differently, then either the system has a sudden demand or the unit is starting to fail. If you are doing normal computing the system appears to normal other than the power supply fan, I would first check in Task Manager if anything is causing the system to have an abnormal loading. If that appaears normal, then consider the power suplly needs replaced.
HERE IS THE MANUAL for that model. The Convertible Minitower and the Microtower appear to use a near normal power supply while the Small Form Factor is not. Here is a power supply for the Convertible Microtower (CMT) series
Thanks for advice. The fan spins very fast even if there is no heavy load. I found a new psu so lets try it. Just wondering, the fan problem happened after i tried to connect a joystick port to motherboard's 15 pin male connector titled 'com B' ? Just coincidence?