I do not own a mid drive but when stuff like that powers down but then comes back up again, I immediately think it is a loose connection. I know that's probably very obvious, but before you do anything else, look at the wiring from the controller to the motor and the motor to the battery. If it is older than a few months there might be wear and tear or water intrusion.

You also might want to find a forum that is specific to that brand of motor and brand of bike to see if it is a common problem.

I'm definitely no expert but I've owned a mid drive for a few years. It could possibly be the nylon gears transferring power from the motor to the bike's gears- that happened to me early on. I'd go full throttle from a dead stop and eventually the gears sheared and binded.

Otherwise, depending on the actual sound it could be the driver fets or the motor itself. Can you post a short video or audio clip of it happening? Also does it only happen under load or can you trigger it with the back wheel off the ground?

Post pics, more details, and probably make sure your cables are securely connected.

Check all the connections, sounds like a loose wire.