May not help a lot but my psu hardly ever spins up its fan and is fully modular. Might be worth picking up a used higher end PSU.
XFX XTR 550W has hybrid fan mode. Not sure how much they are though. Or the Superflower Leadex 650W.
The Corsair RM series fan doesn't even spin until high load and even then it's a quiet fan, so they have considerably put some focus on improving sound levels with their latest ranges.
Dont get a corsair RM series,get a SF leadex 650w,fully modular,cheaper than the corsair,better quality than the corsair.I have 750w fan never turns on unless under prolonged max gpu/cpu load
Too late only had it a few days and it's out and packed up and soldif your happy with the psu you have (except the fan noise)
you could save your self some cash and hassle by replacing the offending fan