maybe a NOTAR (No Tail Roter) type helio.. been around for years.. has the
twin boom you describe and has a "ducted fan" type to control the Tail...
hence the sound you may be hearing..

BT

"John Ross" wrote in message
. 3.6...

I live in San Diego, and besides flying, I like seeing what is passing
through the sky when I am grounded. I keep a pair of binoculars by my
back
door, and go running out when I hear an aircraft passing overhead.

Several times over the last few months, I have heard what sounded like a
small bizz jet overhead, and gone outside only to find that it is some
kind
of a helicopter. I can hear almost no rotor sounds, just the light roar
of
a jet engine. The only distinguishing feature (to my eye) is that it has
two parallel vertical tail structures, kind of like on an A-10 Warthog,
but
very much smaller.

It looks like a normal commercial 'copter, other than that. Does anyone
know what kind it might be?

Thanks,
John Ross

ps. No, it's not one of them U.N. Black Helicopters.