I am hoping to find an answer here as I assume there are many professionals on these forums who may have run into this issue before.
While editing a video I noticed a very high frequency whine that could be heard behind the dialogue of the subjects. I opened the audio file in Audition and found a narrow band of variable frequencies in the 40k hz range. While in Audition, this whine could not be heard, which makes sense since 40k hz is well beyond human hearing. I deleted that band of audio and playback was fine afterward.
I don't want to stop there, though. I want to know if anyone knows what could cause this issue at all, just in case there is a way I can avoid it in the future.
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One cure for this is to stop recording at 96k!
As to what causes it - there are several possibilities. I have found on several occasions that anybody running a video display with a powerful backlight system is likely to radiate a lot of electromagnetic energy at the backlight power supply switching frequency, and anything at all sensitive in the near vicinity is likely to pick it up. They often radiate at lower frequencies as well; I've had an iPad that was being used to display a score radiate at 19kHz. Being an EM source it's not audible, but still annoying.
The other favourite is lighting rigs - they can radiate all sorts of rubbish. Get one of those too close to a mic (doesn't matter what sort, but condenser mics tend to be worse) and the preamp will react to it. The reason that condenser mics are worse is that they all have a very high input impedance at the pickup point, and this is far more susceptible to external influences.
However you look at it, it will be an environmental pickup of some sort.