What is the source of a rattling noise in front of your 1998 KIA Sephia when you go over a bump?
Loose: ball joints, stabilizer links, struts come to mind.
There are a few things that might cause an air conditioner to make a rattling noise. It could be that the there is dirt and debris in the cabinet. If this is the case, cleaning the cabinet should stop the rattling. The reason could also be loose panels on the cabinets. The fan could also be bent and no longer straight.