I don't have the answer, but I have seen that as well and maybe a upsweep sound. I've seen signals that go from side to side almost like they are digging a hole here and then over there, thats a really non technical description, but thats what I have seen on the 7300.
There is another signal I have seen, it sounds like a buzz saw and it looks like the thread of a bolt.
It would be interesting to know the source, and I would guess that the signals need not be local, they maybe coming from a distance.