Sibilance

Sibilance sits in the upper mids and highs, and microphones exaggerate it when you work close in or point them straight at your mouth. Bright condensers and heavy high-frequency boosts later in the chain make it worse. The recording fix costs nothing: angle the microphone slightly off-axis so you speak past it, and hold a consistent distance. Where it is already baked into a file, a de-esser reduces the offending band only when it spikes, which is far gentler than pulling the whole top end down. Be sparing. Over-treated speech goes dull and lispy, and that is more noticeable than the original problem.