It generally happens to SMPS of local quality after 3 + years....
The reason behind that is....as the SMPS gets older..more heating issues arises in it..as a result the Fan is forced to rotate @ higher rpms...thus causing more vibrations to the PSU (SMPS).....hence you get sound which exist when fan rotates at higher rpm...Also adding to it...if in case your SPMS screws are not tight....more noise will be produced....
Its totally your decision to purchase a new one...or wait till its dead
All i can do is suggest you a SMPS to fulfill your needs