Is there something I can buy/make to quiet the noise from an outside central air unit?

Answer 1

You should also have a technician come to tune it up. The noise may indicate a problem such as failing bearings or a loose fan motor.

Answer 2

If this is your property, you can build an enclosure around it. It must be accessable for maintenance if needed and should be able to "breathe" (perhaps made of panels like shutters) It cannot go over the top of the unit. This will block a considerable amount of noise.