In cool edit pro, you can use the noise reduction feature.
Effects, noise reduction, noise reduction. You can then create your own 'noise profile' and use that for filtering. For the profile, choose a section that has the hiss but NO other sounds or music. Highlight it and click Get Profile from selection.
Now you've generated a noise profile specific to your noise. Load that profile and listen to your music. You may not want to use it at the 100% level - while it will reduce the hiss completely, it may also have some effect on your music. Trial and error.
I find CEPs help to be very good for understanding what to do - spend some time in there!
For a less precise but easier way, there is a straight hiss reduction feature Effects, noise reduction, hiss reduction