Hello, I have a question for those who have experience with monitoring screw compressors. The compressor is a ZR7 with a 4 lobe male and 6 lobe female, with an operating speed of approx 7034 rpm.

During data collection I normally see the pressure pulse frequency of the compressor, but at times I notice that the 3rd or 4th harmonic of the PPF is sometimes 2 to 5x higher than the fundamental. I suspect this could be due to some local natural frequency in the machine being excited. The amplitudes of the fundamental PPF was 0.15 mm/sec rms and the 3 rd harmonic varies between 2 to 3 mm/sec rms. I would very much appreciate some opinions as to what could cause this and is it considered normal.
The intent of this question was to try and get a better understanding of what can cause these 3rd and 4th harmonics to become higher than the fundamental. Any information shared would be greatly appreciated. thankyou