Use linear switches
Choose switches with smooth factory lubrication
Lubricate switches properly
Use a softer plate material
Add plate foam
Add case foam
Use a gasket mount
Use a thicker PCB
Use PBT keycaps
Use thicker keycaps
Tune stabilizers
Lubricate stabilizer wires and housings
Use screw-in stabilizers
Install a dampening mat
Reduce spring ping
Use a heavier keyboard case
Use a lower-pitched switch
Avoid clicky switches
Avoid thin ABS keycaps
Try a silent or muted linear switch
Adjust typing force for a softer sound
Test different foam combinations
Use a well-built custom keyboard
Replace rattly parts
Ensure all screws are evenly tightened