I have been struggling with audio hiss in my interviews. It happens in either channel. Especially with soft spoken subjects. I've worked the lav microphones as close as possible, I've traded from wireless to wired mics, and I've tried many menu settings but I still get hiss that I have to extract in post. Here are my current menu settings for audio input:
- -41dBu left and right
- Limiter off
- Mic AGC low
- AGC link seperated
- 1khz tone off
- wind filters off
I also shoot with a Sony PMW Ex1 with similar settings and I have none of this issue.
The first and simplest thing I'd try is to do a camera reset. I'm not familiar with the F3's various menus but the ALL RESET function is probably in the OTHERS menu. See if that solves your audio issue before you have to dig deeper.