Modal frequencies are calculated from the body dimensions, supports, and material density and stiffness (Young's Modulus). If you change any of those values by a factor of 1000, you will see a significant shift in the first natural frequency. If you change the supports, such as a beam supported at one end versus both ends, that will also have a large effect on the first natural frequency.

Find a textbook example of a modal frequency calculation. Build a model of the textbook case and demonstrate that you get something close to the textbook answer.