At flowrate q1, you might have no vibrations.
At flowrate q2, you might have high frequency vibrations, but the amplitude probably phases in and out. E.G. V ~= sin(x)*sin(10*x).
At flowrate q3, you might have high frequency vibrations, but the amplitude stays constant. E.G. V ~= sin(10*x).
At flowrate q4, the vibrations might completely phase out altogether.

Harmonic analysis will tell you if you'll have problems at q1, q2, q3, or q4, operating ad infinitum. However, you clearly wouldn't use Harmonic on q1 and q4, and I'm not sure offhand if CAESAR natively handles q2 harmonically, but I do know that q3 "should" be case scenario compared to q2, which I know CAESAR can handle.

Transient analysis will tell you if you'll have problems going from q1 through q4.

However, you can't begin discussion of your analysis until someone tells you what's going on with that compressor with any amount of specificity.