Samuele,
What is the frequency of the peak and what are the modal frequencies above and below that?
Here is an article that does a good job explaining experimental modal analysis. Figure 20 give you a hint on how Harmonic Response (FRF) of the structure could have peaks that are shifted from the modal frequencies.
https://www.uml.edu/docs/s-v-jan2001_modal_analysis_tcm18-189939.pdf
I just realized I gave you this paper before in your other discussion.
You are navigating away from the AIS Discovery experience