20 in Box fan has secondary hum

Most fans have a secondary hum aside from the normal wooshing sound of the fan blades. What can be done to get rid of hum? Changing speed only makes it louder or subdued. This is not my exact Lasko fan These are the 20 in window kind. Before mentioning cleaning this is a problem with a brand new one I just took out of the box.

I thought it might be some small modification of the blades (?)

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Hi @loninappleton

Does the sound occur if you manually rotate the fan blades?

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Thanks for answering.

I wanted to ask before I took the new one's grille off. But I have another of the same brand I've had for so long I've fixed the handle that fell off. It does not have the secondary hum or at least not to any degree. That's why I thought there might be some hack for the blades. I'll take the grille off the new one and see if it's hitting anywhere but I cannot see any obstructions. Will report back anything I noticed.

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