Reinforced concrete floor (40 m² span) swings slightly

Reinforced concrete ceilings are allowed to bend and thus also to vibrate - depending on the usual spans, also 1-2 cm. Since in residential construction the dead load of the ceiling is the highest load, a resonance of the ceiling caused by "jumping" should not occur. The ceiling thickness of 18 cm sounds realistically dimensioned - although of course reinforcement and concrete quality are also important. As described by jamguy, I also recommend paying attention to spalling and cracks on the underside of the ceiling. Hairline cracks are normal (up to 0.2 mm). If there are larger cracks, a development can be easily checked by applying plaster markers exactly over joints - simply apply a small thin lump of plaster. Even more precisely this can be done with a crack monitor. If the plaster marker breaks or the crack monitor shows changes, a structural engineer should be consulted; I would not recommend supporting the ceiling without professional advice. Without wanting to make a remote diagnosis, I consider fatigue of the floor construction - i.e. insulation - the most likely cause of the vibration.