Sounds like the outer edges of the tires are “Feathered” (high-low-high-low) between the outer edges of the treads. Placing a straight edge ruler along that edge should be obvious if it’s that severe.
This can be from lack of proper rotation or severe worn bushings in the suspension. (rare)
But some cars like my 300ZX Twin Turbo and some other high performance cars have
uni-directional tires which can’t be Rotated because the manufactured tread pattern.
An “Out of Balance” problem would usually make the Steering Wheel shudder/vibrate at higher speeds.
A Engine Belt slipping is more of a “screeeching” noise and usually only is noticeable during starting and acceleration, not consistent speed.