Doesn't the engine drive shaft run through a gear box, with drive line change to the prop?. Might be the gearbox winding down?
Correct. Basically the reduction gearbox that normally sits on the front of the Allison is mounted where it normally sits in the nose and the drive shaft extends from the crankshaft output on the engine to the gearbox. I would say the noise from the gearbox is absorbed somewhat when mated to the engine so I believe it is the gears you can hear.
The Auster should be recognised for what it is: a gentleman's aerial touring carriage and a nice aeroplane.