During a few brief periods there is a training done the Dutch Royal Air Force in NATO exercise location around Schiermonnikoog. These exercises are held predominantly during the off season.
The training will be announced on the defense website well in advance. Whether or not the military exercises can be heard on the (tourist) beach, or in the village of Schiermonnikoog has to with many different factors, including wind direction and wind speed.
You can find more information about the military exercises on and around Schiermonnikoog at: https://www.defensie.nl/onderwerpen/vliegbewegingen. For questions or complaints, you can also visit the same website and/or call the number: +31 (0)77 465 67 67