Audio echoes for a few seconds

Every so often, sometimes once a day, sometimes two or three times in ten minutes, my audio will reverb at 100% for about 3 seconds. It's very random, with no clues it's about to happen.

This has happened while watching youtube, while using Reason, and while using Logic Pro X, so I don't think it's the applications. It didn't start until Catalina.

The system, an iMac (Retina 5K, 27-inch, Late 2014), is running macOS 10.15.3, the sound is going through a Focusrite Scarlett 18i8 (first edition), which I've been using for years now with little problem. I don't have any other devices, virtual or physical. All inputs and outputs go through the focusrite, and the microphone is selected off, and nothing is registering on meters.

Does anyone have any insights? It's not the end of the world, but it certainly is annoying.

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 & 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.