Have you stablished the speed the fan is running?
What do you mean by that?
Do you know if its running at 1500rpms (its full rated speed) or the unit regulates it?
you could try to undervolt it to 7V or 5v
How would i undervolt it? Dont really know stuff like that
Some options,Zalman Fan Speed Controller FANMATE-23" Fan Speed Reduce 3 Pin Power Cable Adapter for Desktop PCZalman ZM-RC56 56 Ohm Resistor Connector
Running at lower RPM can this be an issue for the effective cooling?
Could be, but in a lot of prebuild cases, they prepare for 7200rpms drives, that you dont need as much cooling with 5400rpm, but it really depends on each enclosure and drives. The difference isnt mind blowing though, its 1300rpms vs 1500rpms, that if the specs from the manufacturers are correct. Another 92mm that has lower rpms is the Scythe S-FLEX SFF92A - 92mm Silent Fan - 1000 RPM - Sleeved
If you want to go with similar RPMs fan, then look into SPCR's Fan Round-Up #3: 92mm Fans
, one of the recommended was the Nexus 92mm Real Silent Case Fan - Sleeved
I was thinking of placing -my own design- a quad of smaller fans, just so that all four fit into the dimensions of the big fan. Maybe smaller ones make less noise? Is this possible?
Bigger would end up better, as they would need less rpms to move the same amount of air than a lot of small ones, also smaller are much harder to find quiet ones, while 92 is not super easy but i think would be easier overall.