I redid the ventilation scheme: I completely removed the case fans, removed the drive bay 5.25, turned the power supply upside down, removed half of its case along with 80x80 and fixed my 4-pin 92x92x25 fan, set it to 1000 RPM in the BIOS for both fans. It became much quieter. The air sucked in by the processor cooler is quite enough. G4600 barely reached 45 degrees.
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