Indeed, as mentioned here, feedback is a classic but still limitless source for inspiration and experiments. I like to mix different ways of creating a loop, whether it's a mic, or feedback inside a mixer, especially if it's a stereo signal - one mic against two speakers, or multiple loops inside a mixer sounding in different channels. It's always very exciting to control the sound by changing the mic in space, varying its character dramatically from a shrill shriek, to a deep drone. I, like many, am a big fan of using a flanger in this context, however I'm already somewhat bored with the standard sine modulation in this type of effect, although on vocals it always sounds amazing still.