I purchased this PCIe to SATA adapter card, not realising that the built-in fan is extremely loud.

The 3d printable bracket uploaded here allows for attaching a potentiometer to the card, in order to vary the fan speed. I am using a RV24YN20S module (the 100 Ω version, though I wish I had gotten the 500 Ω version, to make it even quieter).

I am using white tack to hold things together with the friction fit.

The included openSCAD file can be used to tweak the design to suit your needs.