Do you hear a rattling sound every time you turn the PC on ?

The power supply fan and CPU cooling fan are usually the main sources of computer noise. Fans and hard drives also produce vibrational noise that's magnified by the PC case or the surface it's sitting on

If the computer noise disappears after some time, it is probably coming from the fan in the power supply. David Stone of PCMag suggests that the rattling sound from a cooling fan usually indicates that bearings are wearing out

Recommendation: If you're not planning on getting a new computer soon, it's best to replace the power supply sooner rather than later. Be sure to buy a power supply that has sufficient wattage to handle all of your PC's components.