In Captain America: The First Avenger, Howard Stark introduces Vibranium to us, saying that it's 'a lot stronger than steel, and a third the weight, and it's completely vibration-absorbent'. This is while Steve is trying out the Vibranium shield.

And we all know that when you hit something, the reason it might make a sound, like the ringing of a bell or the sound of a gong, it's because hitting the object causes it to vibrate, thus causing the air around it to vibrate, and so we perceive this as sound. For a bell, the surface is smaller, so it vibrates faster (higher frequency) so the note we hear is higher. For a gong, the vibration frequency is lower, so the note is lower.

Then in Captain America: The Winter Soldier, Bucky Barnes punches the shield in the fight sequence on the bridge, just after he shoots Natasha. And the shield made a sound very similar to a gong.

How is this possible if Vibranium is supposed to absorb all vibration (being completely vibration-absorbent). The shield should not be making any sound at all when hit...