Do you mind having two different brands/types of fans in your system?
If you do the fan then I'd suggest for both radiator and overall system performance the Noiseblocker Pro. They're industrial looking so will fit the theme of the 900D which has a real industrial aesthetic. They have excellent noise efficiency. They do as a result suffer in radiator performance and your temperatures will increase with a fan like that, but they will be quieter.
If you simply want a high performing radiator fan that matches the rest of your system then the EK Vardar is definitely a good choice for the entire system. They're not too expensive, easily sourced, and are quiet. If you have the clout the new Corsair ML120's are supposed to be good, but I'd still favour the EK's. You may also be able to find Gentle Typhoons in some iteration. I don't remember who is making them and selling them right now, but they're a classic fan.
If you don't mind having two different types of fans I'd suggest the EK Vardar for the radiator and something like the BeQuiet Silent Wings 3 for the system fans. They're incredibly quiet whilst being really sexy. If they had better air pressure specifications then they'd be the best all-round fans after Noctua. They're definitely more suited to system fans, though, or heatsinks. Low-FPI radiators will work too, but not the radiators AIO's use as they require high speed, high pressure fans.
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