Corrected entry: Howard Stark says that the shield of Rogers is made out of a ''completely vibration absorbing'' material. However, when Carter shoots at it, it has a very distinctive echo to it. Since sound is nothing but vibrating air, the shield is vibrating.
Friso94
20th Sep 2011
Correction: Vibration absorbing doesn't mean there is no vibration, only that it is absorbed. In the same way shock absorbers on a car don't stop the shock of a pothole to the suspension, but absorbs and softens the jolt, vibration absorbing would soften the impact and mitigate the vibration.
rswarrior