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Cue the arm up and while the platter is spinning and gain up enough to hear. Gently blow on the arm to transit it over the platter. Does the noise change? If so, you may have an intermittent ground fault inside the arm. If the noise is stable, repeat the test with the platter off. Does the noise disappear? If so, you may have component in the motor going bad or some kind of short between the motor and the arm ground.
the system is
Amp-Air Tight ATM3
Phono Stage Step up-Conrad Johnson HV1a
Phono Stage-Custom Air Tight with separate tube power supply
Digital-Krell Ref 64 DAC
Krell DT-10 transport
Sony HAP-z1es music server
Speakers-Celestion System 6000
Sub Amp-Sony TA-N77ES
I typo'd, the sound isn't a hum, it is more of a buzz ,
the arm wand on my Graham was replaced after having a ground lead issue in my prior wand
I just opened up the Turntable Power Supply and didn't see any leaking things on the circuit board.
the TT sits on the top of a Roomtunes rack, on top of a marble slab which the TT comes with, and on top of a large Bright Star Audio sand box
you said it hums only when spinning? if the motor is not running, can you hear the sound? if not, can you spin the platter and hear it. If no, and a belt drive, disconnect the belt and turn it back on. if you hear it when the motor is not running but spinning the platter by hand, could be a bearing issue. If you hear the noise when motor is running but belt disconnected, it could be motor bearing or capacitance issue with the motor. I had bearing issue with a gyrodec and when i replaced, it solved the noise issue. trying to isolate to a section of the table will be key to troubleshooting the problem