I would download the trial version of
Vegas Pro. It's a multi-track A/V editing suite made by Sony.
If you can, get the camera and go back to the same location and record the hiss again, like j0n3z3y said. Then open your video in Vegas and also open the second video file. Mute the video track from the second video and then check "Invert Track Phase" on the audio track from the second video. Hit play and see if that eliminates the hum. If it still doesn't or it's still too loud, zoom in time-wise until the time markers are measured in FPS. Then move the audio track around and listen to make sure the second audio track is a complete 180 degree phase shift (where the hiss goes away the most).
If you can't do the above, still open the video in Vegas, but click "Track FX" on the audio track and you should see an "Audio Track FX" window with three tabs. Click on the track EQ one and drag the 1, 2, 3, and 4 dots around until it sounds like the hiss has gone away the most. Ignore all of the parameters below and just do it by ear (if it sounds bad, don't do what you're doing; if it sounds good, do it more). This option isn't the best as cutting out frequencies to eliminate hum will also cut out frequencies of whatever else was recorded.
If you do try to fix it with Vegas and you're stuck on something, feel free to toss me a PM.
Best of luck.