Vibration difference between clockwise and counterclockwise direction

All;
We have a furnace with seven over hung fans bolted to the side of the furnace wall. To reduce a beat frequency every other fan is operating at a different speed, 1080 and 980 RPM. These fans operate in the clockwise and counterclockwise directions and herein lies the problem. All fans run well in the clockwise direction however, during the cycle the fans will change direction and run in the counterclockwise direction. Immediately the 1 times vibration becomes very high, up to 1.04 ips in the axial direction and .8 ips in one of the radial directions. Also, present in the waveform is impacting in the axial and radial directions with a Crest Factor of up to 7.4 Gs. The fan vendor says the problem is structural (a resonance is present). They base this on previous experience but no other supportive testing. I have not had an opportunity to perform a bump test or modal hammer testing and probably will not have an opportunity until January at what time the furnace will be taken down. I simply do not understand the vendor’s reasoning of a resonance issue. The high 1X is indicative of resonance and the Crest Factor needs to be trended and is not reliable.
Other pertinent information;
• Bearings are Link Belt P363
• Belt driven
My questions are as follows;
• Will resonance produce an impact at 1 time running speed in the axial direction?
• Why the large increase in vibration due to the change of fan rotational direction when only three or four fans are affected?
• Is this a structural or a balance/mechanical issue?
Thanks to all for your input and guidance.
Good day,
CharlieRB
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