I'm not sure you can diagnose it much from feeling vibrations if running. As an aside I don't remember it being open to pedestrians - I used to drive it occasionally.

More to the point a proper study a year before the collapse with endoscopes showed the cables were pretty rusted and dangerous https://www.lastampa.it/esteri/la-stampa-in-english/2018/08/...

The company managing it had tendered to get repairs done but they hadn't started by the time of the collapse.