Noctua do make quality fans. How much of a difference it will make though I'm not sure, while their fans are quiet it's not just a matter of fan noise. It's wind noise hitting an obstruction (in this case the radiator fins). Even if the fan were silent x amount of air moving through the radiator is going to creat x amount of noise and there's no getting around it.
If you turn the airflow (fanspeed) down, you'll have slower air and less noise. It also means less air and less cooling. Turn the airflow up and you'll have more airflow and better cooling but more noise. The single radiator is going to pretty much be limited to what it is. There's only so much surface area for heat to dissipate from and the only way to overcome a particular size of surface area is to increase airflow.
In other words while a noctua fan may technically be nicer than the fan you're using don't expect to move 90cfm of air through a radiator and have it nice and silent. There's no magic pill with this setup.
Case fans on the other hand it makes a bit more difference because it's more open air in nature. Then it comes down to the sound of the fan, the airflow it provides and even the 'tone' or sound quality of the fan. That's one area where noctua tends to be preferred. Sound has two properties that affect what we hear, noise pressure (decibels or dba) and tone or pitch.
25dba of low frequency sound like bass is far less 'piercing' than say 25dba of high frequency like a whistle. Think of sirens on emergency vehicles, the high pitched sounds 'carry' further and grab our attention more than medium tones. The sound 'louder' even at the same dba. When it comes to case fans, from brand to brand and model to model some fans have a pleasing tone while others sound a bit more 'whiny' or have an irritating hum. That part of the noise is a bit subjective though since everyone's hearing is a little different and some people have sensitivity to particular pitches that others don't.