Any information on a "humming" moth?

[choksch]

     I am trying to find information on a "humming" moth we have seen here
in South Korea.  It shows up in the September/October time-frame and then
disappears for a year.  My attempts at photographing it have been very
disappointing.
     It is generally between 1 and 2 inches long, has a pair of wings that
beat so fast it can hover (inaudibly) when feeding, and has been seen at
all times between just after sunrise and about two hours after dark.  When
it stops the wings form a triangle around the middle third of its body
(rooted in the middle section of the body but extending back to the rear
section).  The rear section of the moth is striped yellow and black,
similar to a honey bee.
     Four of us saw either a very large specimen (about 4 inches) or a very
quiet humming bird one night.  I have been told there are no humming birds
in Korea and this particular animal made no noise we could hear, but it was
absolutely gigantic compared to all others we have seen.
     Obviously I am not an entomologist.  I have also had no luck checking
in the library here or with any internet resources.  If anyone has any
knowledge of this, I would appreciate hearing about it.
     Thanks.