A Desktop PC built from off-the-shelf DIY parts, rather than a prebuilt from a major manufacturer. These can also be built by an SI using off-the-shelf parts.
When you first turn on your computer there is 100% disk and high RAM usage. Dust and dirt in the vents, on the fan blades could also contribute to the noise. Check the CPU, Video and case fans. Which ones are making the noise? Take the side cover off and look and listen.