garbonzai: [snip]can affect just one small area, f
That's the nature of resonance. There will be narrow points where the amplitude increases and it can be felt, and between them areas with no discernble movement.
Set up a data logger as per @msukiwi and get a week or so of data. Establish what the pattern is/when it happens and it should then be possib;e to correlate that with a source active at those times.