This is one call I had trouble with for years until I was just messing around one day.

Does everyone understand what I mean when I ask, can you make the motor boat sound with your lips?

Using the same amount of pressure with your tongue that you do with any of the other calls, vibrate your lips together lightly and make the motor boat sound.

It still takes a bit of practice, but it isn't as difficult as trying to gargle with the call like I've been told by some of the pros.