Your description matches this https://www.youtube.com/watch?v=wmNrOwFeXsA https://www.youtube.com/watch?v=wmNrOwFeXsA It could be heard just at the edge of the

Your description matches this https://www.youtube.com/watch?v=wmNrOwFeXsA https://www.youtube.com/watch?v=wmNrOwFeXsA It could be heard just at the edge of the normal FM range. Most FM radios cut off before you could reach the frequency, however. The signal was used for something like an early pager service. The early receivers were rather large, making them only suitable for cars. The service was also very expensive, so pretty much only businesses used it for management or sales people.