"Your [item] suddenly vibrates unexpectedly."

e - a blessed scroll labeled XIXAXA XOXAXA XUXAXA.
What do you want to read? [e or ?*] 
As you read the scroll, it disappears.
Your small shield glows silver for a moment.
Your small shield suddenly vibrates unexpectedly.
c - a blessed +4 small shield (being worn).

What happened there? I understand e was a scroll of enchant armour, but the vibration is kinda baffling.

Solution 1:

The message is telling you (in its own cryptic way) that trying to enchant the given item further could destroy it, i.e. you've exceeded the limit where it's still safe to enchant the item (the limit is +3 for normal armor and +5 for weapons and special armor).

Solution 2:

Often, you will see +5 and +7 instead of +3/+5 quoted as the limit. That's because a blessed scroll can take a +3 armor (before) to +5 (after) safely. "Can" means it can also result in +4, in which case some people drain-life it to +3 and try again.