Static developing on audio when screen recording with Quicktime

Hi Guys,

I am using a MacBook Pro 15″, late 2013 model.   2.3 GHz Intel Core i7, 16 GB 1600 MHz DDR3, NVIDIA GeForce GT 750M 2048 MB.  I am running on El Capitan.   I am using Quicktime to capture Screen Recordings with audio using the built in mic.   Initially the audio starts off fine but if the recording is long, I’ve noticed that after 10 mins or so static starts developing on the audio, which gets worse as the recording goes on.  Nothing changes in the environment that I am recording in to explain why this would be happening.

Any ideas?