My air conditioning unit operates in a strange noise. What should I do?

If the noise comes from the inside fan of the indoor unit, a small solid piece might have fallen and hits the fan blades when operating. If you have access, you may use a vacuum cleaner in order to take out the small piece. Another cause might be the loose connection of the plastic parts of the indoor unit (panel etc). Concerning noises from the outdoor unit you should contact with a specialized technician.