How can you reduce the noise coming from your outside ac fan?

It probably just needs cleaning. Simplest way is to take the cover off of the unit, the sides and top come off as a unit and hose it down at a car wash. Wash both sides of the coils being careful to spray the water straight in so that the fins do not bend over and the fan blade. Let it dry for a day before plugging back in and using. It should be quieter and cost less to run.