I am looking to purchase a central air conditioning system for my
smallish home in San Francisco. Because I live in a quiet
neighborhood, I'd like to keep the peace by buying a quiet AC system.
I've got an estimate for a Lennox system (that's the model of furnace
that I have), and the lowest decibel rating is 67 dB.

Are there residential AC systems that make 60 dB or less noise? I'd
like to know about them -- links to Web references and specs, along
with any caveats specific to those units (like they max. out at a
certain tonnage, etc.)