I added a case fan and it is running at 1750rpm constantly and is pretty loud. How can i get it to run slower? And i plugged the 3 pin connector from the fan into the sys fan port on my motherboard. Is that right?
Unless it is a 4 pin fan connected to a 4 pin slot there is no way to adjust the speed of it. It can use all the power it receives and therefore it wants to.
You could probably buy some kind of fan controller that would be able to send less voltage down the 12v and more on the 5v parts of the cable to reduce the voltage level going to the fan (the result would be that it doesn't have enough juice to spin as fast anymore), but that would require additional hardware and possibly some fabrication.
Otherwise, you could just buy a different quieter sort of fan. The wider the fan is, generally, the slower is spins, so the 200mm's are usually quieter than 120mm's which are quieter than 80mm's, etc.
Within each size there are some that are quieter than others as well (ceramic bearings vs additional bearings, etc). Research and guesswork are about your only hope in this regard as for getting a successful result.