飛韻余兄説得對. Electrical wires had 50 or 60Hz and noise. They have their own harmonic frequencies. Mechanical objects had their own resonance frequencies. In simple, motor picked up harmonic frequencies and transfer from direct drive or belt to platter, phono cartridge picked up and amplified. The harmonic frequencies and resonance frequencies could mixed and new frequencies occur. i.e. Receiver intermodulation. On the side, high end turntable platter had drill holes for dynamic balance due to moment of inertia to damp the resonance frequency.