The following text is my expanded notes from my 28 February 2008 interview with Alliant Techsystems' (ATK) space launch systems' vice president Mike Kahn at the company's clearfield offices in Utah.
The [Ares I crew launch vehicle] resonant burning, oscillation is due to the variation of the width of the thrust channel. The exhaust gas flow through that varying pitch generates perturbation. Garry Lyles [NASA Marshall Space Flight Center's engineering associate director] will issue a report [on oscillation] at the end of March. [Question is] will the shaking attenuate with the upper stage, what is the upper stage's natural frequency? The first-stage natural frequency changes as the burning goes on. The [first-stage's] frustrum could be a dampner.
Caption: A CGI movie of what could be the Ares I-X test scheduled for April 2009
Note: this is not an ATK movie I found it on Youtube and thought it applicable