Ghost in the machine?

Several possibilities here (forgive me if some seem terribly obvious).

1. You might be playing multis that use some of your own feedback patches. IS the Microkorg even multitimbral BTW? ...But then of course you would know that you are playing multis.

2. A better guess this time. A built-in mic. THe one that's used for Vocoder. Could you by any chance be getting real feedbacks from that?

3. SOmething is ****ed (OS or hardware). Before assuming that its hardware though do a hard reset. Check the manual on how to do this part but before you reset the Korg don't forget to backup your patches.

If none of the above helps then it might need repairs.

cheers,