Could it have possibly been gusts hitting the prop? This would give the
deceleration and some changes in sound, similar to what you describe-at
least with the prop out front. Was it a gusty or thermally day? A little
thermal turbulence will hit the prop from all kinds of angles changing its
sound.

I always look for the simplest (and cheapest) explanations first.

mike regish

"Peter Duniho" wrote in message
...

Well, we'd only climbed a few hundred feet when something in the airplane
started making a very strange noise. My friend's wife heard it too. It
was
sort of a rapid "growling" sound, lasting a second or two at a time, with
maybe five or ten seconds between. It seemed most likely to be coming
from
the engine, but it was subtle enough I couldn't rule out some sort of
airframe flutter. It might have been my imagination, but it seemed like
whenever I heard the sound, there was a little deceleration from the
airplane.