A grinding noise in a desktop computer is often related to moving parts within the system. The most common culprits are fans and hard drives, as these components have mechanical parts that spin at high speeds. In this case, the hard drive is more likely because fans typically produce a consistent humming or buzzing noise when they fail, rather than a grinding sound, which is more characteristic of a failing hard drive with moving platters and read/write heads. Also, a fan issue would likely have been visibly noticed during the inspection.