If noise of a rocking piston is a concern in the garage in Indiana consider running the pump only at night. With your plan of diffusers and water depth running the pump 6-8 hours a night should be very adequate for turning over the deep water basin of your 1.2 acres. Consider upgrading from a 1/3 or 1/2hp to a 3/4hp that will provide more air volume and faster turnover.
Or build a cabinet and put sound deadening foam in it. Remember to put a fan in there that will suck fresh air through the cabinet to keep it cool. A cabinet with sound deadening foam is no louder than someone talking dB wise.
