Fan-Motor System resonance

[COLOR=#000000][FONT=Tahoma]Hi everyone,[/FONT]

[FONT=Tahoma]I'm a student who's doing an internship with IMEC, Belgium. I'm studying a fan powered by a motor. This fan-motor system goes in a resonance at a frequency between 30 and 40 Hz. The operating range of this fan is 30 to 40 Hz so we can't just skip these frequencies. We've tried multiple things to get the system to stop going into resonance but nothing seems to be working. We've already realigned the axis with the motor. (Motor and fan are directly coupled) We also tried replacing the impeller with a new one, replacing the bearings, adding strengthenings to increase the stifness and we even put better vibration dampers under the installation. I dont know what I can do more, so I'm asking you guys for help. Feel free to comment with your findings. Thank you in advance![/FONT]

[FONT=Tahoma]Arno Vonck[/FONT][/COLOR]