If you have SoundTrack Pro, SoundSoap or any of several other available audio specific programs you can use their noise reduction tools to help reduce the hiss. Most of those tools work by sampling a small section of the unwanted noise by itself, then using that sample as a filter to cancel out the noise during the remainder of the clip.
You could try an EQ filter in FCE and play around with the frequencies until you find the right combination - but that might also affect the audio that you want to keep.
-DH