Do you remember the day that you purchased your current workstation? Maybe it was last week, last month, last year, or several years ago. At one point, it was pristine, untouched by the demands of the workday, but those days have long since passed. Over time, your computer collects all sorts of stuff on it, from unnecessary files to outdated, unused software.
For the average PC, most of its noise comes from its fans. Computers are typically built with cooling fans that are meant to operate at acceptable noise levels. Although, as machines age, they tend to require more work out of the installed fans in order to keep the components cool, which may explain why your old machine is much louder today than when you first got it.