id try bracing the guitar in a way that the jack can not move and trying it. if feedback lessens then have the jack repaired/replaced.
Go for a walk. U will be healthier for it and u wont hear the feedback
cheap pickups, guitar pointed at the speaker of the amp which causes the pickups to router the sound throught the pickups over and over causing feeback. Most likely, cheap pickups.
I'd kill for that sound.