Hello hayesreddoch;
Model # of that Rosewill fan?
If you have it connected to the PSU directly it's going to run at full speed all the time.
To get the fan quieter - its going to have run slower (and move less air).
You can do that with a fan controller. Is your case fan compatible with any of your motherboard fan headers?
Also you could buy a 'fan controller device' or your could add a resistor in the power line so the fan gets less than 12Volts.
Examples: 7V and 5V
-> If it's a cheap fan you might be better off getting a better (& quieter) fan.