It sound like the vib prob I had on a 67 Porche after changing to some after market mags.
Assuming that you have changed tires and had multiple unsuccessful balances. You have to go technical; ie, one varible at a time.
What wheels do you have? Pin drive? or tapered lug nut/hol;e??
Have you been able to pin point a side? Try switching left to right, etc see if it causes a change(I doubt it)
If all negative, take tire off and put rim back on and torque down. Put rim back on the spindle and take dial indicator reading for roundness off the tire bead mounting surface(not the rim edge), thats what centers the tire mass. At the same time check the rotor for out of roundness in it mounting holes.
This is hard to describe in a few minutes in writing. If you don't follow , send PM.
Bottom line on the Porch vib, it was cause by the mounting lugs being engineered poorly, they did'nt pinch/self center, but were a cylinder that went thru the wheel. This cause the tolerance stackups between the wheel /lub combo to go up to .015 -.020 in. That was enough to cause the vib on a "light " weight Porche
The solution, as it was, was to mount the rims hand tight, spin the wheel with an indicator and find the high spot, rotate that spot down,,lower the car to weight that spot. This took out the out of round tolerace of the crappy nut disgn and torque down.
Most of the time the vibe was gone after that. I found that it was amazing that .015 in could cause that! I was told by dynamicist that what .015 was causing a "resonance" at a specific wheel frequency. Thats why it sppeared suddenly and would go away. Mine would appear at about 64 mph and be going away at 70 mph. It set my cruise speed alway at 70 or above!!
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