Here is an example of the sound that was heard on the radio for approximately 2 to 3 minutes that was broadcasted on the Dutch radio.
https://on.soundcloud.com/QAUa2Kkgef1gDxDQ6
To my ear it sounds like AFSK, kind of like the Bell 202 scheme. Here's the first passable search result I found with a clean recording: https://www.youtube.com/watch?v=-PXxSHGrF-8
There's only a 3 second sample out of the 2 minutes of radio programming.