The slim fan in question says: Noise: 28.89 dBA Airflow: 38.05 CFM. So im thinking thats why its loud
Well, ok: seemingly you misunderstand how fans works and what they rating means.
Those you quoted above are (some of) the declared maximum specs. That is, according to the manufacturer, the fan while spinning at 1600rpm will emit around 29dB. And will emit all that noise because it will move a lot of air (and that's probably what it's happening). You may give a look at your Tri-Cools data here
for a quick comparison.
The issue is this fan (actually any fan) is not meant to run at max speed every moment: a) because it's noisy; b) because it will wear out faster.
Why does it spin so fast? If I got you, because you connected it the wrong way.
This fan is meant to be hooked to a motherboard header, not to a PSU: and it does need to be connected to a motherboard voltage control header.
If you do so, the motherboard will dial up and down the fan speed according to a specified temperature: otherwise, the fan will run at full speed everytime.
That's what you're actually experiencing, I guess.
It doesnt sound like something is broke, sounds like its just spinning fast. Its connected via a 3pin plug to the PSU. I have 4 (Lets call them A,B,C,D, E) of these 120mm fans all set at lowest level...
And that's the error: you managed the slim fan like it were a Tri-Cool fan, and you shouldn't.
Actually you're running the slim fan as if the Tri-Cools were set at an higher setting (please set those Tri-Cools to that setting and let me know whether they are loud or not).
The difference is clearly in the three-positions switch: the Scythe has not because it relies upon the motherboard control (as either your CPU fan, or as any other fan but not those Tri-Cools), which you omitted hooking it directly to the PSU.
I guess the solution is to connect the Slim fan to a proper voltage control header: your motherboard has three, please refer to page 33 of your motherboard manual for details.