The new roofers are plenty quiet. Here is one on Craigslist:
New Carrier Roof Air - $820 (Alachua)
Date: 2011-10-18, 7:15AM EDT
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15K BTU HIGH CAPACITY HEAT PUMP-AC,SUPER LOW PROFILE CARRIER. $820 obo.
Carrier's most powerful and compact roof mount air conditioner/heat pump. This new design is very compact, energy efficient, aerodynamic and stylish looking . At only 7" tall you will barely see it on the roof. Complete with ceiling panel and remote control. New in box with warranty. (We purchased it new, and due to the nature of our bus's framing, it would not fit where we wanted it. We had to go with a basement unit instead.) This is one of the quietest roof airs I have ever heard. We are near Gainesville, Florida. Come take a look and make a reasonable offer. We can bench test it for you. Call 386 418-1251