It's going in a Drobo, a five bay external drive unit. There is no rig, there is no PC motherboard logic, there is just a fan with a 12v feed, no speed control.
It still has components over and around which the airflow has to pass, in order to cool them... In this case, a stack of hard drives, so not as noisy as a radiator, but also not whisper-quiet. Most will be good enough for your noise requirements, though.
Do you know
which model of Drobo you have?
I don't know if the circuit board (whatever kind it is, but it
will have one) has a 2, 3 or 4 pin header, the last two of which may well mean it can control the speed anyway, and I'm surprised that it doesn't already, TBH... It may depend on which model you have, but there are lots of posts on DroboCommunity about replacing the fan, which is either too noisy or has failed just outside of warranty. If yours is fairly new, as in 2015 or later, it appears they did have temp sensors and some method of speed control.
However, a 3-pin fan will often work on a 2-pin header, and you can either cut and solder the relevant wires together or use a converter cable if it doesn't. You could even twist/braid the wires and tape them up, if you wanted. Sometimes you get lucky and you can just pop the pins out of one connector and they'll fit the other easily.
Interestingly, several people seem to have used the same Noctua fan I mentioned above.
http://kevinrye.net/index_files/drobo_fan_replacement.php
http://blog.netscraps.com/tips/replace-drobo5n-noisy-fan.html
https://drobocommunity.m-ize.com/t/5n-fan-replacement/141892/20
https://drobocommunity.m-ize.com/t/5n-fan-replacement/141892/22
https://technikblog.ch/2011/03/howto-drobo-lautlos-machen/
https://www.ifixit.com/Guide/Drobo+FS+DRDS2-A+Fan+Replacement/78662
^ A couple of different models there, one of which looks to have a 3-pin header as well.
Whereabouts in the world are you, Feek?
The fan that's in there is running at 2200rpm, there's no control. Whatever I put in place will run at full speed, I just want it to be quieter.
That's the thing, though - Any 120mm fan running over 1,000rpm will start to get a bit noisy... which is often barely audible in a PC, but still too much for some people... while any 120mm fan running at 2200rpm will be
******* loud. Fans like that are intended for things like servers, which are usually in a server room, insulated to keep it cool and consequently noise outside that room is not a concern... which is why server rooms (that use small fans) are often deafening. Large fans moving slow are better and quieter than small fans moving fast.
I thought four pin fans had to have some control, you can't just chuck 12v up them, is that correct?
Only if you want speed control, but you can still run them flat out with 3 or even just 2 pin headers if they're wired right.
3-pin fans also have speed control, but they do it by varying the voltage. 4-pins use PWM to do it -
https://www.overclock.net/forum/246-air-cooling/989566-pwm-fans-what-does-mean.html