I read a guy turn his the other way round so it was sucking air from the inside and getting a huge noise difference, so wondering if there's any other random tricks before I try.
Put a resister in the cable. Replace the fan with a quieter one. I've done both and as long as it's a decent PSU you should get away with a slower, quieter fan. No guarantees though. 9 years ago I bought a Seasonic P1000 and have used it for for 99% of the time with the fan permanently switched off in my PC in my sig. It cost a small fortune at the time but quality counts.
I'd say the list of what we cannot do are your best options. Clean is your best option if the fan has changed the noise profile, new fan for best performance, new PSU if you want no fan. Otherwise your other option is to divert the noise, without overheating your PSU. Flipping the fan is one option if it doesn't mess up your case air flow, which is true in most modern cases. A decade ago maybe more people messed around with PSU mufflers, foam lined boxes to absorb the fan noise. This fell out of favor as new fans achieve higher results with out comprising air flow. Edit: Wait I forgot one... custom water cooling the PSU. I do not recommend this in any way.
If you're going to take it apart to clean it or change the fan don't go poking around with anything conductive like a screwdriver or your fingers though, don't want you dying on us.
Pretty much. I'm assuming this is the fan noise you're talking about, in which case turning it round so the fan faces in may help as the noise is directed inwards more and has to bounce around to get out. A bit of foam or rubber underneath to dampen vibrations is an option.
