My mucus plugs tend to be drier than the usual stuff I cough up during a vest treatment or neb. Because they are usually the last thing to come out after treatment for an infection, they can be a different color and consistency. They vary in size and I can usually "taste" on my breath that it's on its way up, or feel it as it becomes dislodged.
Yes, this is an icky description, but with CF you run that risk <img src="i/expressions/face-icon-small-smile.gif" border="0"> I'd compare the feeling of coughing up a mucus plug to that feeling you get when you blow your nose and finally get that thick piece of snot out of your nose that you can tell has been blocking your sinuses for awhile. (I hope that made sense)
Happy coughing everyone, and don't forget those mucolytics!