It seems to be a packet based digital mode. By its packet length it is definitely closer to Pactor ARQ, not Amtor ARQ. However it lasts a bit longer than Pactor. Also it it has an ACK waiting period that's much longer than ones like Amtor or Pactor who's waiting slot is barely wide enough to fit the ACK reply in. Several such reply packets could fit into the wait period in this mode. My first thought was maybe packet radio, but HF packet is 300 baud, but this sounded slower, like Pactor-1 ARQ in 100baud mode. So I took out MultiPSK and tried to decode it with it set to Pactor, but (as expected) it didn't decode it. The packet was just too long for a standard Pactor packet.
Also it wasn't in a ham band, but had a center audio frequency about 1.4kHz when my Icom PCR1000 was set to USB and tuned to 13.055MHz (putting it's center RF at 13.0564MHz, well outside any ham band). Thus it's likely to be for some professional communication equipment, and may not be a standard mode for ham operation.
Here I've posted a download link to a zip archive containing three MP3 files. One is a recording of the signal I am talking about, and the other 2 are Amtor ARQ and Pactor-1 ARQ (which I downloaded from a radio sounds site, and then trimmed to make a reasonable length). These 2 files are just a reference.http://www.mediafire.com/?h1tn9xnbk58dco6
And here's a zip archive containing the spectrograms of each of the 3 signals that were in the above zip archive.http://www.mediafire.com/?9lm5l9y0agx730t
If someone can tell me what this signal is, I'll be really happy. I've been hearing it now for years on my ICom PCR1000 receiver and have no clue what it is, and that's just driving me nuts.