Many thanks to SWLing Post contributor, Carlos Latuff, who shares the following:
I bought a small analog radio and I’ve been listening with it.
On September 4, around 17h48 UTC, I heard a strange signal on the band of 13 kHz, since it’s not a digital dial, I don’t know precisely the frequency but maybe if your readers may listen to the signal they may tell what it is about.
It almost sounds like the preamble to an SSTV message, but I’m not sure about that noise that’s in between. Readers, if you can help Carlos ID this signal, please comment!