+1 Post a sample.
Is one camera set to manual level and the other auto? Auto level will add gain (hiss) until it reaches a predetermined recoridng level, so when its quiet the hiss ramps up making it more apparent.
If you are serious about audio there are three possible ways forward:
To record audio seperately on something like a zoom h4n or similar (has built in ok mics or can take good externals)
To record audio seperately on something like a tascam DR-60D which also acts as an interface with the camera, so you have the security of the audio in two places, on the devices own SD card and in the cameras SD card as part of the movie file. (no built in mic, externals only)
Finally, an interface only device, such as a beachtek or juiced link. (no built in mic, externals only)
The main benefits of each of these approaches is more control over levels, cleaner sound and headphone monitoring. The Tascam & zoom stand out in this regard as you are actually listening to the recorded audio that will be on the Tascam, there is the the potential for there to be a problem between the beechtek or juiced link and not realise as you cannot monitor from the camera.