I have recently pick up an old turntable (my detailed setup goes below) and facing the background noise issue. I think it is something being picked up by my tonearm cable because when I move it or touch it - the sound changes. Also I found out that when I unplug tonearm cable from the preamp, the noise reduces and disappears completely when second channel is disconnected.
Here is what I hear from the loudspeakers (volume really cranked up - you can't hear normal record at such volume). Recorded using my phone put close to the speaker (ogg file):
My setup goes as follows: Mains 230V 50Hz Turntable - Denon DP-47F, 100V japanese version, plugged through transformer, not connected to mains earth Cartridge is Goldring E3 (MM) Preamp - Cambridge audio Duo, connected to mains via earthed cable Tonearm cable - noname, how well is shielded is unknown. L/R channel + and - as well as ground is soldered to PCB under tonearm inside the turntable. Amplifier - I use receiver Yamaha RX-V467, not earthed to mains.
My best idea as for now is to use some braided copper sleeve like this: https://www.amazon.com/Tinned-Copper-Braided-Sleeving-Diameter/dp/B01MEG9V2R/ref=sr_1_17?keywords=tonearm%2Bcable&qid=1571934888&sr=8-17&th=1
Put L/R cables inside this and replace the ground turntable cable with it - solder it to the ground connection inside the turntable and also connect it to ground terminal on the preamp.
If anyone can give some advice on that, I'd be really grateful.