I use audacity and use noise removal to get rid of un wanted hiss, i'm asuming you are wanting to remove hiss??
Highlight about 5 seconds of the audio which has nothing but the noise you are wanting to remove, go to effect at the top, scroll down to noise reduction, click on get noise profile, highlight the whole track, go back to noise reduction, adjust the bars to how much you want to remove and click ok....hope this helps!