This is the Advanced Narrowband Digital Voice Terminal used by the US military for secure voice communication on HF. It's heard a lot, usually associated with KY-99 or other voice encryption modules, during exercises and drug operations. It's controlling more or less the same 39-tone modem seen elsewhere in our NATO spectrogram, but in a different manner, using a pilot tone and more complex preambles.
This particular transmission is using ANDVT's "short interleave" mode (MIL-STD 188-110A). The various synching tones last around six tenths of a second, after which we see all tones initiate and begin to transfer data using tribit (8-phase) TDPSK keying. This creates the hissy, buzzy, noise associated with 39-tone modems. The red line near the bottom is the pilot tone, which makes this ANDVT mode sound a little more beepy than the NATO burst plotted. At the end there's another short beep as the interleaver is flushed, but we don't see this here.
All plots made with GRAM.EXE.