Thanks for your replies. I didnt think it was reverse thrust but I was unsure to the correct term.
It was only the 3rd and 4th flights on a turbo prop and, being more used to Jets, just wondered what it was. I love the noise made by a turbo prop, although I did notice that sat behind the engines is quite a lot noiser than in-front even with the Q400's anti noise/vibration system.
Paul