There was something like this a while back... was someone screwing around and definitely not the USAF. But that was around 6925, not way up on 6988.
Next time you hear this on 6988, check the other HFGCS freqs to see if its parallel with them (4724, 6739, 8992, 11175, 13200, 15016).
Could be related, unrelated, or who knows what.