@Erin Weiss
Could it be bees in the wall? I have removed hives 4 studs wide and 8 feet high into the ceilling. Take a stethescope and lightly and I mean lightly tap on the wall to see if the noise pattern changes. It might also be carpenter ants as they will make that kind of noise (like crunchy cherrios) both of these insects could be in the walls and you would never see them inside the homes (even if there are tens of thousands) as they are just nesting there and foraging for food outside.
Of course because the noise is by electrical panel that should be a concern and evaluated by a licensed professional. Just check the outside to see if you see any insects flying and or walking to towards the home on a consistent basis (check power lines for crawling carpenter ants that will often trail along them this time of year) . The insects I mentioned could be entering at a point a great distance from where you are hearing the noise but they will crawl to an area that is "safe" and nest (they love electrical panels).
Bats could be the noise but normally they roost at hight points of the home at the rake boards and or soffits and sounds more like shuffling.
I was thinking water and or plumbing issue but if you had the water off for an extended period of time so I am guessing that is not it.
