These passive noise reduction devices are normally used in large industrial and agricultural applications but would they make a difference if used in PC's? There are also large custom made discharge cones for industrial fans that also act as mufflers (and look vaguely like straight through mufflers I've seen used on dirt bikes):
https://www.assemblymag.com/article...uction-options-for-axial-and-centrifugal-fans
https://www.assemblymag.com/article...uction-options-for-axial-and-centrifugal-fans