Water has not be helpful for me and coffee make makes it worse. Some of the talents I use to record will make this noise and some not. Some are not even conscious that they are doing it and some keep trying to hide the gum they were chewing when they came. But most of them will thank any advice that will help them to improve their performance. So my advice is to keep making them aware of this, I´m sure it can be avoided with practice.
If this doesn´t work, nothing makes my mouth drier than brushing my teeth.