Three pumps sat beside one another on a concrete foundation. Two of the motors on the pumps operated with low levels of vibration. The third motor had high levels of 120Hz vibration. As with the previous case, when the good motors were put in the third location, the 120 HZ vibration was high. A resonance test was performed and it was discovered that where the vibration levels were high that the foundation had a 120Hz vertical resonance that was not present under the other two motors. When the foundation under the bad location was jack hammered out, it was discovered that the reinforcing bar had not been welded as designed.
The last case shows that even though things may appear the same from the outside there can be unseen problems that can affect either the process or the dynamic tuning.