Ryzen 2600X, ASRock 450M Pro4 motherboard, Asus STRIX-RXVEGA56 graphics card,
Deepcool Captain 240 EX 240mm Liquid CPU Cooler, Corsair CMK16GX4M2B3200C16 Vengeance LPX 16 GB., Windows 10 home edition. The PSU is about six months old.
Its such a 'mechanical' sound, I can't imagine it coming from anywhere else.
Many high-efficiency PSUs have a relay to bypass inrush limiting resistors/NTCs once the PSU is powered up to reduce losses, so a click when the PSU turns on/off would be expected. Unless there is something weird going on with AC power, there should be no reason for the bypass relay to click on/off during normal operation.