Neighbor left this note under my door. What should I do?

Our downstairs neighbors are in their 70s, and we've received 3 notes from them since moving in January. We often run into them and they’re very nice in person, but I’m unsure what to do. We can’t exactly stop walking around or closing doors.

The building is poorly built, so even the smallest movements cause vibrations, like when someone opens the front door (there are only 4 apartments in the building).

I’ve apologized in person twice, but it’s starting to feel a bit excessive. Any advice on how to handle this?