Juular

Could be something as simple as bad/oxidized contact somewhere between cables and PSU/motherboard terminals. Have you and OP tried to reseat them ?

Sounds almost exactly as my Seagate Exos 10TB, i was baffled when i first heard that, never in my life i've encountered a HDD this loud. It also makes the same sound when it spins up after spinning down due to no usage, so i had to disable the HDD sleep timer because this thing kept waking me up at night, the PC is right to my bed.

Yeah, but how would it end-up on top of the PSU casing ? That's obviously an AIO coolant leak, unless maybe GPU capacitors blew up ?

Thanks for a good laugh ! Just registered to tell you that there's no transformer liquid in SMPS (there is in industrial oil cooled transformers tho), and as Jon remarked, if that were electrolyte of failed capacitors it would've been much smaller amount of different composition, brownish color, fast drying, not oily viscous fluid as on your photos, that's with 99% certainty a coolant of your AIO. Deepcool AIOs with this exact configuration (tube going out from the pump block and back) are known for leaks, check everything again, you must've missed something.