Power vent noise

Hey there
I have a question about a new power vent we installed for a customer yesterday, I got a call from the homeowner complaining about the humming noise the vent makes and that it keeps him up throughout the night. I installed a Lomanco powercool 2000 and I have never had this kind of complaint before. The homeowner is willing to pay for any changes I make to quiet the noise. Has anyone ran into this problem before? Is there any other manufacturer who makes a quieter vent? My thoughts was to install a !/4" sheet of siding insulation under the vent to absorb the vibration. Any other thoughts or suggestions

Perhaps some of the interior bracing is loose.
You will always get a little bit of hum out of a power vent, but it usually can’t be heard inside the house.

I have never heard of such a thing. Is the substrate solid or it there bad plywood?

Is the vent loose?

Something just does not sound correct.