There are a couple ways to deal with hiss but it depends on a couple factors. Signal to noise ratio and if you have some areas of just hiss, or the noise. Years ago I bought a program for $50 or so bucks called Sound Soap. If you have Final Cut Pro studio you can use sound track pro to achieve the same results with it’s noise removal. What you end up doing in both cases if find a second or more of just the his and background that you want removed and it samples that noise. Using that information it filters out the noise from the audio. If you have a lot of noise then your audio left over will sound a little strange.
I have used these programs before with great success as long as I had good noise to sample and my original audio was recoreded well.
I own a 7D and I after looking at using devices that allows me to record the audio to the camera I decided to use a separate capture device. I bought the Zoom H4D I think the model is and it worked great and much lower noise then those device’s that allow for internal camera recording.
-Lars