I replied to your question last night, but it hasn't shown up. Strange. Anyway, here we go again.
The bearing/bushing of your PSU fan is probably becoming worn out. You can fix it for some time by peeling open the sticker covering the center of the fan and applying a couple of drops of machine oil on to the bearing.
But this is really a temporary solution. It may last for a few weeks or a few months. It's best to replace the fan. The worst-case scenario is that the fan may stop working without your knowledge and cause the power components in the PSU to overheat.
Depending on where you buy it and the quality of the fan, a new one will cost approx. Rs.60-150.
Unscrewing the fan and screwing a new one on is very easy. But the connecting wires of the fan in many cheap PSUs are soldered to the circuit board instead of being plugged in. That may be a problem if you are not familiar with soldering, but it's a simple job for someone with soldering experience.
It is also possible that the fan is rattling because the fastening screws are loose. You can check this easily with a screwdriver. But a worn-out bearing is more likely since the rattling stops after some time, i.e., after the speed has stabilized.
