Floor vibration/suspension behavior
I have purchased a hone last year and are experiencing vibration/suspension behavior around the kitchen island whenever the kids run around the area OR someone takes quick or big steps. The items on the island rattle and make noise especially the steel or glass items. When one is sitting in the couch or on the floor in the family room and someone runs or takes a big step, you can experience the vibrations or feel little movement in the floor, like a small suspension.
The whole family/kitchen/dining area is a 25x25 open floor layout. We have kitchen island with granite top, dish washer, garbage disposal and that's where the issue seems to be. I checked with the builder and initially after taking a look they had some cross-sectional pieces connecting the joist so that the weight is distributed across the joists right below the island. This did not help and they have expanded this solution to cross connect all the joists and this did not address the issue. Finally they came back and said that its a "suspension bridge" like characteristic and is by design and there are no issues structurally.
What are my next steps and can I have this evaluated with a regular home inspector OR do I need to go with some other structure related company. Please advice...