Frequency of Macbook fan sound
Screen recording definitely taxes laptop graphics cards and makes the fans whir. There's no easy way to avoid that.  I would instead try to use a mic that gets as close to your lips as possible (without popping), and move the laptop as far from your mouth as possible (external monitor / keyboard / mouse). The results will be better than trying to use noise reduction to improve the audio afterwards.  Noise follows an inverse square law, so a small increase in distance from your lips will result in a big decrease in noise, if your mic is much closer to the desirable source than it is to the undesirable source.