When the phono pot is open, especially wide open, and the needle is on the record, and not moving, there is vibration we cant feel or hear, but its there.
You say when you turn the TT power off on your NuMark it goes away. Guess what? When you turn the power off, the transformer is off and transformers hum, and make small amounts of vibration when powered up. So, the needle which is VERY sensitive is picking up this tiny vibration, then your phono pre/mixer is amplifying it, and now YOU hear it.
This is why the High End turntables pay so much attention to such teeny weeny vibrations, and thats why they cost as much as they do.
Back in 1986 when we got our rubber band suspension console I couldnt believe how much quieter, and cleaner my records sounded, and with our original console which wasnt rubber band suspended, there was no rumble. But, there are tiny vibrations we cant hear, but cloud the music, etc, etc.
Mr. Scott Fitlin