If you can make a moan, you can produce the "Herrr" portion of the "Herrronk" the same way. With a call which is tuned very easy to blow, sometimes is easier to make the moan (Herrr portion of the honk) with your hands open, rather than cupped over the exhaust end, as usually is taught. Opening your hands lowers the back pressure and helps keep the moan from unintentionally sliding into a cluck on a light tuned call.
Ever hear the sound that a big dog makes when it sees something it doesn't like, or is unsure of, a deep, throaty, low volume growl? Try to make that sort of a sound while you blow into the call, then bring your tongue up quickly to the roof of your mouth to make the cluck and cut it off. Once you have it .... play around with some variations to produce what you feel is the best sounding note.