Noisy Split Airconditioner (or any induction motors) caused by power line harmonics

The noise sounds like beat frequency of 50 Hz and 44 Hz - a modulated 50 Hz (or 44 Hz) by 6 Hz envelop (Click here to hear). I tried to google and found no satisfactory explanation. The 44 Hz must come from the rotation of the compressor as it is driven by induction motor (a psc or permanent-split-capacitor motor actually) while the stator magnetic field rotates at 50 cps (1500 or 3000 rpm), the rotor rotates at 44 cps maintaining slip to develop torque. But why it wasn't that bad before?

To make it short, I found the answer after 2 months of investigation: it was caused by harmonics at the power line. I needed 2 months because I don't have proper tools to measure the harmonics, rms, refrigerant pressure, etc, all come in a sort of improvisation.

The remedy came after a lot trial and error, an inductor in series with compressor motor is the answer, I made a hand-wound inductor using an old transformer core, I don't think my cheap chinese DVM measured the inductance properly, it read 15.4 mH . Now the sound is like this, not just the noise is abated, the current drawn is also lower (measured by that DVM, average measurement).

Here is the inductor:

• Core came from old 220V to 15V 5 Amps transformer
• Core cross section is 4 cm x 2.9 cm
• 120 turns (I made 27, 54, 81, 108-turn taps), originally 108 was the final tap but turned out more inductance was needed and I only managed to add 12 turns without removing the bobbin from core.
• Here is the voltage waveform across the coil, the 3rd harmonics (150 Hz) is clearly shown
  

Vertical is 5 V/div, Horizontal is 5 mSec/div

• and here is the current across sensing resistor (R=0.047 ohm) in series with the coil

Vertical is 100 mV/div, Horizontal is 5 mSec/div

I should've captured the oscillographs with slow shutter speed, but I only realized it after, and it was taken outside the house in a bright and humid 34 C (92 F) day so I don't want to do it again. I do have the video of those oscillograms if a need arise latter.

Now how do people describe the sound of that noise:

• beat frequency noise?
• fluctuating noise?
• low frequency noise?
• rough noise?
• pulsating torque noise?