I investigated the DARM displacement noise from 40Hz to 50Hz.

Fig.1 showed the spectrogram of the DARM displacement. The amplitude of the 46Hz (and 45, 47, 54Hz) have time variance. Also, it has some coherence with each line amplitude.

Fig.2 showed the IXC microphone, there are some common lines, but amplitude variation seems independent.

Anyway, we should turn off hte air compressor (and if possible air cooler)