What are "haunting" tones? was a "short" by Rick Beato that I thought was a beautiful exercise. 
When a composer writes an etude it usually is musical and helps us work on technique at the same time. My favorite etudes are ones that sound like music. 
In this video I explain how to play this exercise / etude that Rick Beato played and showed in 55 seconds. He published another video explaining the exercise but I've got a couple of changes in the placing of the notes that make it easier to play.
It's best to use an electric guitar or a guitar with a cutaway to play this exercise where I show it.
Hal
https://youtu.be/ebsjmIuGy3k