What is this knocking noise that seems to be coming from behind or above the cabin filter?

Sounds like a damper door maybe fluttering or you have an engine knock it's right behind that metal wall right behind that white plastic That's your engine sitting right there and transmission that seems to be happening offly fast just sitting there with your ice and whatnot off so I would say it's the damper door or something stuck in the fan box that's blowing open and closed partially the damper door is stuck but it's loose in the position it's stuck in so it can flap back and forth with the fan on or something like that pulled the air filter off pull the air filter tray out you might hear better and be able to see put your hands on the plastic and try to get your hands close to the vibration or listen with the stethoscope to where it gets the loudest or generally it will be right behind that piece of plastic