Icing accumulation will generally cause a slow increase in vibs, but since ice is shed in chunks, the
shedding may cause a sudden vib increase.
Keep in mind that both (or all) engines will pick up ice at about the same rate, but will probably shed ice at different times.
If nil is found looking elsewhere maybe a screwed up vib detection.
tion
This is always a possibility, and was not uncommon a decade or two ago. I should hope vib detection is more reliable today (i.e. fewer false positives)