you may be dealing with something that was 'borderline' ok, and now problematic.
only phono input?
as mentioned, move noisy side left cable to right input, see if noise moves with it. if so, problem is prior to pre-amp.
if noise stays left, problem is not tt or cable, problem is preamp.
after that fundamental answer, we move on. if prior to preamp, don't forget the wires/connectors onto the cartridge pins.
if preamp, switch tubes l/r, noise move?
I have one tube socket that very occasionally makes noise, just the slightest tap of the tube, noise gone for months, a loose pin sleeve in the tube socket I believe. so switching tubes, the problem might disappear, then reappear.
easy enough to move a set of non-problematic cables to the phono temporarily, and temporarily run a separate wire for the phono ground, just to rule the cable out.
only phono input?
as mentioned, move noisy side left cable to right input, see if noise moves with it. if so, problem is prior to pre-amp.
if noise stays left, problem is not tt or cable, problem is preamp.
after that fundamental answer, we move on. if prior to preamp, don't forget the wires/connectors onto the cartridge pins.
if preamp, switch tubes l/r, noise move?
I have one tube socket that very occasionally makes noise, just the slightest tap of the tube, noise gone for months, a loose pin sleeve in the tube socket I believe. so switching tubes, the problem might disappear, then reappear.
easy enough to move a set of non-problematic cables to the phono temporarily, and temporarily run a separate wire for the phono ground, just to rule the cable out.