My car has a natural vibration at the same frequency at 55mph. Just enough to notice.
It does it not matter what rear gear is in it, nor matter what tires and no matter the pinion angle, etcetera. That vibration is related to the chassis, and after a few years of looking for it, I just accepted it.
but then, I got me an overdrive, and I don't drive 55 anymore, lol, so I quit caring. Cruizing at 100, she's smooth as glass.......... but the wind-noise is nearly horrendous, lol.
What I did was;
I went for a road test. I ran her up to 65mph, then shut her off. As she came down thru 55mph, there it was.
Next; I Jacked the car up, ran her up into the vibration zone; then started taking things off.
Wheels off, no difference. Drums off, no difference. Driveshaft off, no difference.
So, in my case, it was in the body.
Some years later, I read that it was a common characteristic of our classic A-bodies.
When I got to thinking bout it, I remembered that just about every A-body I ever had, 67 to 75, yes, they all did it.
My 1980 Volare (F-body) did not. Dads 84 New Yorker did not. And my 65 V100 wagon, being a distant memory, I couldn't remember.
I had a 69 Barracuda with a slanty/904 and super big tires on it, that on one 600 mile trip, just kept putting me to sleep at 55. I thought it was those tires. wou-wou-wou-wou down the road we went. 600 miles there and 600 miles home.
When we got back, I installed some nice rally wheels and regular radials on it, and there it still was. It hadn't been the tires....... Daymn those N-50s on adapters looked so good back on there.
I installed the new radials on the front.
Darts and Swingers, same
Dusters and Demons, same
Barracudas, same
wou-wou-wou ............... just noticeable.