This is something that can happen if the computer does not have built in noise reduction. What is happening is that the computer's speakers are sending out sounds, and the microphone is picking it back up and sending it back through the computer.
There are a few different solutions that you could try in this situation. The first would be to ask participants who have this issue to wear headphones. The second would be to try third party software that would cancel out this noise automatically like Krisp.ai.