In order to get this example of feedback I ran my suitcase model Rhodes, which I confine to my studio, through an outboard processor with some nice amp modeling and cranked up the gain. For the processing I used an eleven year old Yamaha A3000 sampler. The A3000 allows you to edit and apply processing to an incoming signal and it has some pretty nice sounding effects. Once I had adjusted the processing I held down the sustain pedal on the Rhodes and let the amplification do the rest. Tapping or gently knocking the instrument also produced some nice sounds. Here’s a snippet of the results.
Rhodes Feedback
Over processing usually leaves you with audio that lacks it original luster, or perhaps it starts to sound like the processor itself. However, sometimes you might end up with something interesting as a result of pushing the processing beyond the normal boundaries. While listening to the 
To generate this pattern I loaded a basic 
There are so many ways to use processing to make scary sounds that it’s almost too easy. The classic reverse reverb in the original Poltergeist comes to mind. This example is a recording of a conversation with a colleague during a lunch break at a busy sandwich joint. It’s been reversed, pitched down significantly, run through a low pass filter, slowly phased, filter delayed and run through a long reverb. All this processing has diffused the voices into a hellish ambient drone. 
I came across this old late night session of sleepy Rhodes melodies and decided to render about fifty eight seconds of it. It was originally recorded on November 3, 2006 at about 12:53am. I love electronic timestamps.