|If you have enough dirt and dust accumulated inside the PSU the fan/s may make some odd noises.|
Use canned or compressed air to blow out the PSU from both ends. This will be messy, best to do outside. While you have the case open blow out the rest of the computer. DO NOT use a vacuum cleaner.
After blowing out check all cards, RAM and cables to verify they are fully seated.
Use proper anti-static precautions when working inside the case. Static discharge can destroy sensitive electronics. That is why you should never use a vacuum cleaner.
If the PSU is still noisy after that then you should replace it.