At test pilot school my professor used to say that "flutter" is a four letter word. That looked nasty:eek:
 
Search the archives for the word "Flutter" or maybe "Buzz", and you'll find lots of previous discussion - and a few stories of it occurring.

Paul
 
At United we had a pilot write up that the plane had a "flutter" coming from the tail. You should have seen the engineers scrambling. They were about to disassemble the entire empennage. Turned out to be a spherical bearing attaching the elevator that had failed.