Brooklyn,
First you need to identify where the hiss is coming from. Here is how I would try.
First plug the cable going into the computer into a different recording machine see if the hiss goes away. If it does then ts in the computer somewhere, if not, then it's in the source somewhere.
If its in the computer, unplug everything from the computer that you possible can and still be able to run the computer. if it goes away, then plug them in again one at a time until it comes back. You have your problem. If it is still there with nothing else plugged in, try recording with different software, reboot the computer, try updating drivers, do you have another capture card you can check it with, ?
I assume this is that same board you use on air and there is no hiss on air. If so then it is in the output of the board. Try a different output. Try a new cable. Try a different board, or any other source you can find.
Once you identify the source of the problem, you will have a better chance of finding a real fix.
Good luck, let us know what you find out.