Thank you.Here are a few things worth the try:
Add sound dampners from inside / outside the cabinet.
Instead of air cooling go for either passive cooler (noctua etc) or liquid AIO cooling. I would avoid liquid cooling though. If the fan noise is from power supply, get a bigger wattage rated SMPS.
Edit: typo
Will try dampening and the cooling option.
I don't think it is the power supply. It comes on only with heavy CPU usage.
Regards