Sorry if this is a duplicate, I don't think my first reply went through.
I have exactly the same issue with one of my Hall fans. It's amazing how much vibration is transmitted to and through the car by one out-of-whack fan.
I haven't taken the time to diagnose the problem yet. Could be a blade out of balance, or out-of-round, or walking on the shaft somehow, or a bad motor?
In any case, I mentioned my problem to Gary Hall, and even though I bought these fans in 1992 or so, he told me that if I found a problem, he'd gladly exchange a new one to fix it. Gotta love that!
Mike
P.S. I just remembered--I had a radiator fan (four-blade aluminum) come apart on my 427 Cobra replica, and it really wreaked havoc. You'd be astounded at how much vibration THAT generated when one blade broke off. It felt like the whole front of the car was coming apart. Eventually it smoothed out, as the rest of the blades snapped off. I was very lucky that it didn't destroy the radiator....