I suspect you are either using a microphone that remains connected and rests near the monitor or have other speakers connected to your PC or laptop that are powered separately and too close to the monitor.
I personally do not use the monitor speakers. They are just to small for any quality sound. But here are a few ideas . Make sure you have a shielded audio cable ( important ) increase the distance between your computer and monitor . Make sure there are no other electrical devices near the monitor.
I have found that most monitors have inferior speakers so I never use them. I use external speakers that I have owned for years. They are not expensive and you will never have the feedback or inferior sound quality found in the built in monitor systems.
First make sure the audio cable is pushed in all the way. A small gap can cause feedback. Also make sure you have the audio plugged in the right jack on the back of your computer. Newer computers may have more than one audio input jack and the color of the audio cord jack maybe a different color then your computers color coded audio jack. If you have multiple audio jacks on your computer you will have to switch between the jacks until the feedback is gone. If you have a webcam with mic move it around and if the feedback noise changes you will have move the webcam and/or make sure the cables are not twisted around each other. Make sure your audio cable is not twisted around any power cables. Make sure you do not have anything magnetic close to your speakers like a paper clip holder. Hope this helps you.