JeffK says may be a leftover from unexplained ISI trips.  I'll study.

Here are TFs from the Ground STSs to the ISIs GS13 in low gain.  I'll switch to high gain and compare when that transition won't potentially disturb the commissioners.

In low gain, there is good coherence bewteen 2 and ~10 hz.  Cross couple channels, shown for HAM2, are all pretty small.