this strange sound is transmitted on medium frequency at 518 KHZ USB. it contains important safety info for mariners (on board ship you can get a display unit with the RX built in or connect your ships radio to a computer). I don’t always get good reception with my random wire antenna due to interference (should build a ferrite with a tuning capacitor or something like that) but this time NL Coastguard can be picked up for a bit (until the signal is swamped by QRM (interference) 😥 )