Strange sounds

The last couple of days the conditions were quite nice on both 20m en 10m. Last night I heard some VK's and ZL's on 20 meter and this morning the first KH6's.

I made some nice QSO's on 10 meter the last couple of evenings. Today was the first day that the band was also open during the day. There was quite some slow QSB at some moments but I worked some new DXCC entities on 10 meter. I made nice Q's with F5IHP, I3CGV, G4HZW, EA4EUI, F/PH1MRF, F6IRG, UR5KDX and some more.

During the QSO with Bert F5IHP a strange sound was coming up on the band this morning. Bert thought it was some local QRM/QRN but I heard the same sound moving over the 10 meter band. So nothing local but it could be heard across Europe.

A little later (around 8:45 UTC) I made a recording of that sound with my webcam. When you strart the video on the left you can hear it and see it moving up the band. A little later I heard the same thing again starting at around 28.490.

If you have any idea what this is, please leave a comment.