I think what you are hearing are processing-artifacts caused by noise-reduction, they are particularly noticeable in “s”, sounds e.g “Crisis”.
Noise reduction at 10dB is about as much as you can get away with without it causing conspicuous artifacts.
( Typically I use less than 10dB Noise reduction, say 6 ).
A hint of hiss is preferable to the computery artifacts inevitably created by noise-reduction.
The damage caused by noise-reduction is irreversible. Hopefully you still have a previous version before noise reduction was applied.
BTW
There are two free plug-ins for Audacity here : DeClicking & DeEssing , worth having if you do narration.