[en] In 2009, a new 4-rod RFQ was commissioned at the UNILAC at GSI, Darmstadt, and went into operation in 2010 [1]. At high rf amplitudes strong modulations of the rf reflection emerge with ν_m_o_d ∼ 500 Hz. They are attributed to mechanical oscillations of the rods, excited by the rf pulse. As these modulations could so far only be seen during the rf pulse by means of rf measurements, a direct observation of the mechanical vibrations was desirable. Such measurements have been conducted using a commercial laser vibrometer, allowing the investigation of the mechanical behavior of the RFQ in situ and independent of the presence of rf power. Results from investigations under standard operation conditions confirm the vibrations as the source of the modulations observed by rf as well as their excitation by the rf pulse. Further measurements revealed more details about the excitation process, leading to a better understanding and possibly new ways of mitigation.