I can get soft tree yelp like yelps with my Lonnie Gilbert. The outer parts of my striker are higher pitched while the center is softer. I can get it while playing in the center. A big part of getting good turkey sounds that aren't too loud is how you hold the box and striker. I like to hold the striker in my left hand between my base of my thumb in my palm and my middle two fingers. I then hold my scratch box in my right hand thumb in front four fingers behind. The next step is very important, it's how you marry your hands together. I put the base of my thumbs in the palms together. When I do this, the lip of the box is pretty much 90 degrees to the striker. Now you can get nice turkey sounds that aren't loud locator type yelps. The closer you play you the middle the softer and the further to the sides the higher pitched. You can even keekee far out on the sides by locking the box tight and dragging it straight across.
I know there's different ways to run a scratch box. This is how I like to do it. Just keep practicing! It's addicting.