I have also installed a 140mm fan above and have no problems with the cabinet vibration. Check where the vibrations are triggered (directly at the fan or faulty feet, as described above). Put the fan on the table and see if it lies still.

I have not fitted an additional fan, but in my case not all four feet are completely on the floor, so the enclosure may be slightly warped. Your enclosure may also be slightly warped and that is the problem. Or else the fan is really defective.

I have mounted Corsair iCue fans, also with rubbers, and I have no case vibrations. Are the fans possibly defective?