This is a great sound made originally by the beatboxer SidFx. I learned it from his tutorial and wanted to offer my take on teaching it.
It's done by combining an inward tongue roll/oscillation with a special whistle/wind sound.
Take a look at SidFx and his tutorial: https://www.youtube.com/watch?v=D3soI94JRwo. His scratch is the best I've heard - much better than mine. Also you might like his other videos, I think he's great.
