The seismometers showed an increased noise in the Y-direction when put on top of the granite slab. By tapping the slab, you can tell that its really a mechanical resonance of the lead balls + granite system at ~15-20 Hz.
I tried new balls, flipping the slab upside down, and sitting on the slab for awhile. None of this changed the qualitative behavior, although each of the actions changed the resonance frequencies by several Hz.
I have removed the granite/balls and put the seismometers back on the linoleum floor. The excess noise is gone. I have put the new big box back on top of them and we'll see how the data looks overnight.
I expect that we should remove the linoleum in a wider area and put the seismometers directly on the floor.