Our neighbor has complained about a high-pitched noise coming from our
dryer. I've tested the dryer, and it makes no unusual mechanical
noises, and actually seems to be fairly quiet. Today, I even
demonstrated this to the neighbor, and we both agree it must be
something to do with the exhaust vent and/or fan.

Right now, it's got flexible tubing (tin, aluminum?) that bends once
(90 degrees) at the base of the dryer, travels up the wall about six
feet, and then makes another 90 degree bend to the outside. The
outlet on the outside of the building has a simple hood and flap.

Question #1: Could this setup really produce a fairly loud, high-
pitched noise that can't be heard from inside (just the other side of
a drywalled, non-insulated garage wall)?
Question #2: Since I can't hear it from the inside, can I rule out
the exhaust fan and conclude it's a whistle from rushing air moving
through the vent hose?
Question #3: Would moving to a rigid vent pipe or a different outlet
cover help with air/whistle noises?

Thanks for any input. Much appreciated.