2002 BMW 330 Car shakes at 40mph-60mph

I just bought a used 2002 bmw and it came with 20in chrome wheels. The problem im having is my car shakes mildly and the stering wheel shakes mildly at 40mph-60mph. I just had a wheel balance on the bmw but the car is the same they also rotated my tires and put the correct tire pressure in my tires but the problem is still there. Once my bmw hits 65mph and up the car does not shake anymore but as soon as it slows down to those speeds (40-60) it shakes. Please help what is my next step.