Yes, and I'm starting to notice some patterns on the slice of spectrum near 8992. Right after and sometimes just before the Andrews traffic, I see spurts (10 secs or so) of some type of digital data (sounds like bubbles) on either side. Last night, right after the first SKYKING, there were some guys who appeared on 8953.5Khz speaking in a language that I'm not familiar with. I recorded it in case anyone's interested.
There is, and always has been, other traffic around all of the HFGCS freqs, I have heard many languages right on top of or within a few kHz of all of them at one time or another. This activity is not unusual. 8992 kHz, and the surrounding area, is not “reserved” for HFGCS traffic. Some other countries do not recognize the systems use of that frequency, and many, many, more fishermen ignore bandplans and regulations.
The HFGCS is supposed to deconflict operations. This means “other” traffic, besides EAMs and SKYKINGS, should happen between, before or after, the higher priority traffic. So in general you hear phone patches and such when the high priority traffic is complete. Another thing you hear at times is MIL STD 188-141 of various flavors, this is digital data/traffic. While it is not all that common on HFGCS it also is not unheard of.
Your descriptions of “sounds like bubbles” might be a description of MIL STD 188-141 ALE.
Here is an example of ALE, in this case 2 other frequencies, one used by FEMA and the other by the US Coast Guard:
Modulation Example, Digital, MIL STD 188-141 ALE, 12216 and 12222 kHz, USB, April 15, 2013 - YouTube
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