Turboprop noise

Yes, its another engine related thread.

Today I had a tea break at work (wow!) and decided to go for walkies outside. Five minutes earlier they (the guys who actually work where I go during the day) deciced to to do some engine runs on the ETPS Andover.

Now I was standing approx 200 yards upwind and enjoying that lovely turboprop whistle / whine (actually, it was bladdy loud as well !) and got to wondering what actually causes that unmistakable whine.

Its not the prop noise, cos I could hear that while they where cycling the props, and its not the exhaust eflux (big word that !) so what causes that whistle ?

Is it the gearbox between the jet engine and the prop ?

Over to to you guys