Just come back from a flight where my drivers door popped open 100knts 2000ft on a 206B3
The main thing is the banging was very rhythmical to the point I had already entered auto and was taking a good look at the instruments before being happy it was just a door and giving it a pull to test.
We were close enough to the airfield to spin around and pop it back on grass, but with a little pedal it keeps the door from carrying on flapping around until you reach 40knots-ish.
Its only when you are down that the rotor down wash sends the door flying open!
Thought I would share my experience on this old thread as I know it is a common occurrence to many, but for the 1st time "door opening mid flight" experience, reading this thread prior to it happening to me really helped me not to panic!!!