Is the mic always on? Where is it placed? Is it close to your main speakers?
Could be your mic is picking up some interference from the main output.
I’ve had similar issues. I found keeping the mic as far as possible helped and I didn’t have the mic volume past 12o’clock on the knob. And I kept the mic off when I wasn’t talking.
Hoped something I wrote helped. If not, I’m sorry. I hope you figure it out. Have a good one!