The filter in my AC vibrates when it is fully installed. If partially or completely removed, the unit is quiet. Is there a relatively easy fix?

Serveral fixes come to mind,a coat hanger cut,straightened and bowed in a way it puts pressure pushing against the filter would be a cheap simple way ,some install a couple sheetmetal screws fron corner to corner and then stretch a small bungie cords from one screw to the other in a way to hold tight the filter,, not knowing exactly how your filter holder is set up makes it kind of hard