According to my QRH (CFM-56s).
There is an ECAM advisory when N1 vibration is equal or greater than 6 units, or N2 is equal to or greater than 4.3 units.
It states: the (advisory) is mainly a guide to induce the crew to monitor engine parameters more closely.
The procedure states if there is a rapid increase in vibration above the advisory, retard the affected engine's thrust lever.
On ground I would suspect icing, and a run up to 70% N1 should shed the ice. In flight if there is icing induced vibration, the engine can run up to shed ice, in idle descent for example.
Sorry I don't have FCOM 3 to hand to elaborate further.