I finally put your noise together was a running Saab 340. As has been said above, after the aircraft is started and up and running you move the Condition levers above the MIN gate up to the MAX gate. On the ground, this will only take the props out of feather and will have no control over the blade angle once over the MIN gate. Taxi speed and blade angle are now controlled by moving the POWER levers out of the START gate. After you have done all your checks and have returned to your parking spot, the whole sequence is reversed. You now pull the POWER levers back to the START gate. (Now, here comes your noise..) As you pull the Condition levers from MAX to just above the MIN detent nothing happens to the blade angle, 'UNTIL' you pull the Condition levers over the MIN gate. At this point the blade angle will go from Ground Fine ( l ) into Feather ( - ). If you do this quickly, the blade angle will change rapidly, causing the air around the prop to make your 'farting' sound. I was taught when bring the Condition levers over the MIN gate to slowly retard the levers until you see the slight spike in PGB oil pressure. This is the internal piston in the prop beginning to move. Once you have the spike, you stop moving the Condition lever until the prop slowly transitions into Feather. Either way works, neither way causes undo wears. I just think it shows much more care of the aircraft when you do it the way is was taught.
On the Saab you will also hear this noise after the crew has the aircraft up and running and they have do their pre departure checks. Part of the checks is the 'Autocoursen' test. When you do the test, prop blade angles will change quickly from Ground Fine to Feather and will make the same 'farting' sound.
[Edited 2004-07-15 22:25:57]
[Edited 2004-07-15 22:32:23]
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