Zoom Problem:  severe audio feedback or looping that appears to originate with one participant.

There are two possible causes:

1. a participant has the volume turned up very high on their computer which loops back into their computer microphone.  Ask participants to drop their computer volumes a bit.  If this doesn't fix the problem ...
2. a participant has the Audio connected via Computer and has dialed in via their phone.  This creates a nasty audio feedback.  If a participant wants to switch the Audio, they should do that via the Audio icon (the microphone icon below) by leaving their current audio connection and then connecting with the other method.