>Set governors of each to exactly the same engine revs.
>Natural differences will cause a beat frequency between
>the two engines.
>Engine shed will sum air pressure waves and produce a
>high amplitude somewhat subsonic signal.
>Whump a Whump a Whump a .....

I have experienced something similar during the day with a single engine. It
was idling along as it went past my grandfathers house, but you couldn't
hear it, but you could sure feel the air pressure change. I suspect the
engine was idling with the absolute minimum of fuel combustion to keep it
idling over, and this meant the thing was behaving as an air pump, and not
making any noticeable exhaust noise.

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist