My living room dining room have hardwood floors. When you walk across them, the furniture shakes. I though this was because the truss work underneath wasn't enough, I put another row of the X's about 4 feet from the other truss work. That shaking still happens. I realize now the floor isn't shaking side to side, it is shaking up and down.

Does anyone think doubling up the 2x7 joists in the basement might stiffen the floor system a little more? The joist there are about 15 feet from the center heavy joist to the cinder block. I thought if I got the same joist wood and screwed the two together, tight up against the plywood subfloor, it would reduce the rumble effect.

I am thinking doubling up 4 joists might solve the problem.