This could be from a couple different things. It could be that your condensing fan blade is out of balance causing the whole unit to shake. It could also be from the unit being aged, rusted and not as structurally sound as it used to be. Screws are not holding as tight as they used to. Stripped out. Less likely but still a possibility your compressor could have an internal mount broke and shaking the unit. If your unit is shaking really bad it should be looked at. If a condensing fan blade is going bad it could possibly throw apart and damage components inside your condensing unit.