I did Ravana about 124 (+ about 30 unsuccessful attempts) times and seen only one whistle... which was then taken by some newbie who kept dying all the time. The worst part though is to find a good party instead of a bunch of whiners who quit after 3 runs or die a lot. My suggestion is to either wait for the drop rate to increase or farm it casually whenever you feel its your lucky day. It's not really worth wasting hours of your life for a few shiny, colourful pixels.