Looking at both graphs that screams out electrical appliance of some sort.
Items on a fixed timer can often be seen to turn on/off at exact times each day.
The timing on your graphs is a little more variable, but still happening at roughly the same times each day and lasting for a similar duration each time.
Tracking these things down can be extremely difficult.
It can be anything near your line between you and the cabinet.
Water pumps, sewage pumps, treadmills, industrial lighting, generators, etc etc.
The list of possibilities is huge.
Fortunately it only causes the SNRM to drop a small amount.
I see others with similar but the SNRM drop is enough to cause their line to resync each time the nightly "event" occurs.
You could try resetting bitswaps per tone just before it starts and see if it effects any particular tones.