The designer of the ACC should assure that fan pass frequency or fan blade natural frequency does not come within 10 to 20% of the motor bridge natural frequency. I believe the API suggests 20%.
There are two excellent presentations that discuss this topic on the ACCUG website.
The first is by N. Romano which was presented in 2015 no. 3 paper, Wind Loads on Fan Blades and Blade Dynamics. He discusses how not only do you think about the motor bridge but actually the prevailing wind side has downward flow and on the leeward side there is upwards flow. So as blades pass through this region there is a vibration inducement. For this reason, I favor off numbers.
The second presentation by Mr. Scottt Basham presented in 2014 as paper no. 10 named Maintenance and Vibration Issues and Solutions. They also experienced many vibration related trips an discuss how they solved this problem.