Mostly the issue is non-repeatable conditions near criticals.

The phase change with regard to speed relates to the damping present or Af - amplification factor.

Af = (pi/360)(Wc)(d ph/d n)

n - speed in rpm
ph - phase in degrees
Wc - critical speed, rpm
pi is pi and 360 is 360

d ph/d n is the slope of the phase (derivative) at the critical.