Harmonics in the output signal of Flowserve Logix 3200MD

For a PID Controller at Natural Gas Booster Compressor, I have navigated the sinusoidal waveform and the trend that it's producing. Now, in general, in an X and Y axis graph, the setpoint that I've given to the controller, suppose that it is in a horizontal straight line on that graph, the actual value or the output coming from the compressor, that should be synchronized with that setpoint line. I know that it can't be exactly like the line plotted of the given setpoint, and will have some harmonics, but it should be minimum, as to resemble a straight line in this case. Now, when I've looked at the output coming from the compressor, the harmonics are too high with respect to the setpoint. The ripples in the suinosidal wave of the actual compressor output value, are much higher than they should be. I've tried to vary the frequency and gain, which is the pressure (i reduced the pressure), but that only increased the time frame, while keeping the harmonics same. Now, the question is that how can I reduce those harmonics so that the original output value resembles with the actual setpoint value i-e reducing the harmonics? Do I need to change the proportional, which is the pressure and also called as the gain (I've already varied the gain and it reduced the harmonics, but it wasn't enough), or do I need to change the integral gain or the derivative gain of the PID? If there are some other measures that I should be taking, please list them as well. Although,what I've found out so far is that increasing the derivative gain upto a certain extent will help in damping those oscillations while keeping a deceasing grip on the proportional gain, but I'm not so sure. Please help me out. Thank You.