When I recorded with quicktime and the sound was on. These crazy echo noises starded beeping. Check this yt video out: https://www.youtube.com/watch?v=zV6pWEjBw&feature=youtu.be
It's just feedback. It happens when the sound from the speaker enters the mic, amplifying it and repeating it over and over again, causing a positive loop. Just like a guitar pickup does when held close to a speaker.
Frankly speaking, recording with QuickTime is do really not satisfying solution. I did it, only to find that it has no option to record system audio only. Terrible thing! If you don't mind, try another one, like Joyoshare Screen Recorder, which allows you to create scheduled task, cut recording, highlight cursors, set output format configuration......Anyway, in my eyes, it's the best video recorder for mac.