Audio echoes for a few seconds
The tool I would start with is MicroSnitch if no one has a hunch or guess what would enable intermittent sound input. 


https://obdev.at/products/microsnitch/index.html (Free trial and more details here)


Another thing that's quick to check is Siri. That could trigger a short and intermittent mic active event (and doesn't show in the Microphone section/tab of Security &amp; Privacy). Other apps should show in system preferences for all other apps entitled to mic input before you redo or tear down the main sound processing setup.